Just how intertwined are Presidential and Senate results?

Introduction

As any election observer will know, ‘split ticket’ voting is becoming an increasingly rare phenomenon in Presidential years. Fewer and fewer states opt to back different parties for President and Senate with each passing cycle, increasing cohesion between federal races. (Gubernatorial contests have more variance owing to their non-federalized nature, but are few and far between in Presidential cycles) 

A given state’s Presidential result is now more or less the best indicator of its Senate outcome. In other words: concomitant Presidential and Senate races have melded together; the topline is a bigger kingmaker now than ever before.

The table above shows the precipitous decline in different partisan outcome between Senate and Presidential contests in each cycle since 2000. Column 1 lists the number of states backing a Republican Presidential nominee while electing a Democratic Senator; column 2 lists the number of states backing a Democratic Presidential nominee while electing a Republican Senator. 

In 2000, nearly a third of the Senate seats up for election yielded partisan splits. By 2016, that number had dropped to zero. Given the fact that 2020’s single split (Maine) was unexpected in pundit world, the future of simultaneous outcome deviations does not look too bright.

But there’s more under the hood that is best explained by analyzing vote share. Oftentimes, regional strengths allow Senate candidates to outperform the top of the ticket while still losing. On the whole, though, variations in share have declined consistently since 2000, a telling sign of the gradual dissipation split ticket voting in federal races.

Methods

This analysis will be looking at Republican Senate vote shares by state for each Presidential cycle since 2000. It attempts to show how indicative the Republican Presidential nominee’s vote share has become over time in terms of explaining the variance in Republican Senate vote share. 

Put simply, the relationship has grown increasingly well-defined. In 2000, George Bush’s vote share explained about 22% of the variance in Republican Senate vote share. Last year, Donald Trump’s share explained 93% – a stunning increase in just two decades.

2000

In 2000, just 22% of the variance in Senate Republican vote share could be explained by George Bush’s share at the Presidential level. The linear distribution is still positive, but it is not clustered enough to be particularly strong. That tells us that while Republican Senate shares generally increased in tandem with Bush’s share, the topline wasn’t as indicative of down-ballot performance as it is today.

Ten states yielded partisan splits in 2000, a number that has not been matched since. Strong incumbents running in states hostile to his or her party’s Presidential nominee account for most of these occurrences. 

Popular moderates like Jim Jeffords (VT), Lincoln Chafee (RI), and Olympia Snowe (ME) cruised to landslide reelections against weak opponents, each garnering about 25% more of the vote than Bush. 

Strong Democrats like Kent Conrad (ND) and Robert Byrd (WV) annihilated their Republican foes, both of whom ran far behind Bush.

Despite its Republican status at the Presidential level, North Dakota Democrats were broadly successful at winning Congressional races for decades; voters generally reelected workhorses like Conrad, Byron Dorgan, and Earl Pomeroy without much hesitation.

2000 may have marked the beginning of West Virginia’s Republican ascendance at the federal level, but it didn’t disadvantage the “King of Pork” at all. Nothing short of a state institution, Byrd served until his 2010 death. (His 51 year Senate service record remains unbroken)

In the non-partisan Georgia special, former Governor Zell Miller easily defeated Mack Mattingly, reaffirming his earlier appointment as interim Senator. He did this while Bush won the state handily at the federal level, quite a feat from a modern perspective. In some respects, the ascendant Texan’s victory foreshadowed the 2002 midterm defeats of Senator Max Cleland and Governor Roy Barnes, outcomes that would usher in a new age of Republicanism in the Peach State.

Swing states also produced some interesting splits. Bush lost Pennsylvania, but conservative Senator Rick Santorum, ironically a former Pittsburgh Congressman, was able to win an easy reelection against hapless Democratic Ron Klink. Klink actually outran Gore in portions of his native SWPA, but Santorum sealed the deal with victories in blue-trending Philly Collar counties like Bucks, Montgomery, and Delaware.

In the Sunshine State, Bush Jr’s razor-thin victory did not pull Congressman Bill McCollum over the line. He finished behind Bush by vote share, ultimately losing to Bill Nelson. Nelson won the day by far exceeding Gore in Republican Panhandle counties and the burgeoning I4 Corridor.

In Missouri, Bush’s narrow victory was not enough to save John Ashcroft. Likely crippled by the Show Me State’s sympathy vote, the future Attorney General lost to Governor Mel Carnahan in a squeaker. Carnahan died in a plane crash weeks before the election, and his victory remains the most recent instance in which a deceased challenger ousted an incumbent Senator. From a more analytical standpoint, Ashcroft lost because he extensively underperformed Bush in rural, traditionally-Democratic enclaves like the Bootheel and the “Lead District”.

There were also noticeable performance differences among states that backed the same party for both President and Senate. Token Republicans challenging popular Democratic Senators like Dan Akaka (HI), Ted Kennedy (MA), Jeff Bingaman (NM), and Herb Kohl (WI) received significantly lower shares of the vote than Bush.
Well-received Republicans like Richard Lugar (IN), Trent Lott (MS), Mike DeWine (OH), and Bill Frist (TN) easily outperformed Bush against weak opposition. (Tennessee was definitely affected by Al Gore’s ‘native son’ status, though Frist’s popularity probably played a role too)
Let’s not forget open seats either! The retirement of Daniel Patrick Moynihan, New York’s scholarly Senator with a penchant for pragmatism, gave First Lady Hillary Clinton her springboard into elected office. Congressman Rick Lazio lost, but outran Bush. Lazio, a Long Island native, managed to score bloated victories in Gore-won Suffolk, Nassau, and Richmond (Staten Island) counties. He also ran ahead of Bush throughout the upstate, padding his margins.
New Jersey was quite similar to its neighbor. Long-time Democrat Frank Lautenberg’s retirement allowed Goldman Sachs millionaire Jon Corzine to swoop into the seat. He won, but moderate Congressman Bob Franks finished ahead of Bush to keep the race close. Besides benefitting from boosted margins in Somerset and Bergen, Franks outran Bush extensively in the south. In Ocean County, a core element of the state’s Republican base, Gore came within two points of winning. Had he done so, he would’ve carried all of South Jersey.
In Nevada, incumbent Democrat Richard Bryan retired. His move gave former Congressman John Ensign the chance to vindicate himself after a 401 vote loss to Harry Reid in 1998. Sure enough, Ensign triumphed. He finished well ahead of Bush, won every county in the state, and delivered his party one of its two Senate pickups (the other was Virginia).
The biggest Republican disappointment came in the Cornhusker State, where Vietnam hero Bob Kerrey’s retirement yielded what should have been easy pickup opportunity. Attorney General Don Stenberg underperformed Bush throughout the state, ultimately losing to former Governor Ben Nelson.
*AZ is excluded from this count, because Republican Jon Kyl had no Democratic opposition*
2004
2004 saw our R-squared value grow to 33.4%, an increase of about ten. President Bush was reelected over John Kerry in a hotly-contested slug fest, but his vote share by state still did not strongly explain his party’s comparable Senate figures. The steady observed increase in R-squared over the four year period unveiled the writing on the wall: Presidential and Senate results were indeed becoming more intertwined.
In a slight drop from 2000, only seven states yielded partisan splits in 2004. That’s an unthinkable figure for the modern mind to comprehend, but back then it would’ve seemed normal – possibly even smaller than normal. Much like four years before, most of these splits came in states where incumbency trumped the topline.
Judd Gregg (NH), Evan Bayh (IN), Blanche Lincoln (AR), Byron Dorgan (ND), and Harry Reid (NV) were all great examples of strong incumbents reelected easily in states somewhat hostile to their parties at the federal level – a hostility exemplified by the Presidential vote in each.
These solid performances had as much to do with incumbency as they did with opposition quality. Gregg faced a 94 year old Democratic activist, Dorgan trounced an unknown attorney, and Reid crushed an anti-gay bigot. In most respects, incumbency is inextricably interwoven with opposition quality. Parties usually don’t field top-tier candidates against Senators that they perceive as unbeatable.
Pennsylvania and Colorado had closer Senate races, but results varied from the Presidential topline in both. 
In the Keystone State, Bush’s suburban improvements and seemingly-newfound ability to make every state somewhat competitive could not top Arlen Specter. The moderate incumbent had just survived a bruising primary from the right by Congressman Pat Toomey, and ended up getting about 4% more of the vote than the President. Like Santorum, Specter swept the Democratic Philly Collar. He also did exceedingly well in SWPA and the Luzerne-Lackawanna Coal belt, performances Bush could not replicate.
Incumbent Republican Ben Nighthorse Campbell retired in Colorado, opening up a competitive Senate seat in what was then still a mostly Republican state. Attorney General Ken Salazar managed to beat businessman Pete Coors of the Coors Brewing Company. Coors received 5% less of the vote than Bush, underperforming the President in Eastern Plains counties, the Hispano region, and the Denver Suburbs (Jefferson, Arapahoe, Adams, Douglas).
Like four years earlier, there were interesting instances of performance differences in states that had the same partisan outcomes at the Presidential and Senate level. Most of these results can be explained by incumbency and candidate quality.
In Alaska, the issue was nepotism. Frank Murkowski appointed his daughter Lisa Murkowski to fill his Senate seat after becoming Governor of the Last Frontier. She received nearly 13% less of the vote than Bush, narrowly beating former Governor Tony Knowles in what had been a tossup race.
John McCain (AZ), Chuck Grassley (IA), and George Voinovich (OH) were all popular Republican incumbents facing token opposition, making it no surprise that they exceeded the President’s figures extensively. Republican challenges to Democratic giants like Chris Dodd (CT), Dan Inouye (HI), Chuck Schumer (NY), Pat Leahy (VT), and Ron Wyden (OR) were similarly embarrassing considering Bush’s respectable performances across ‘blue America’. 
In Illinois, one-term Republican Peter Fitzgerald retired. The original GOP nominee, Jack Ryan, had to leave the race in scandal during the heat of the campaign. As a result, Republicans flew in arch-Conservative Alan Keyes of Maryland to face off against a youthful state Senator named Barack Obama. He got destroyed, winning only the most Republican counties in the Land of Lincoln and receiving nearly 18% less of the vote than Bush. (In a sign of the old coalitions, Obama actually won southern Gallatin by more than northern DuPage)
2008
In 2008, our R-squared finally exceeded 50%. The writing was definitely on the wall at this point, but there were still fascinating instances of variation from the norm. As usual, most of the split partisan outcomes were precipitated solely by strong incumbents in states that would otherwise probably have backed the opposite party for Senate.
By 2008, Susan Collins was coming as close as she could to rivaling Olympia Snowe. Moderate, but not quite liberal like her colleague, Collins had perfected the traditional New England Republican brand. Even in a year as change-oriented and anti-Republican as 2008, the Pine Tree State’s junior Senator cruised to victory over Congressman Tom Allen. She carried every county in the state, garnering 21% more of the vote than McCain. (The Arizona Senator carried just Piscataquis)
The other powerhouse incumbents were red-state Democrats. Jay Rockefeller, the protégé of one of America’s richest families, seemed farther removed from the Mountaineer State than pretty much anything. Nevertheless, he managed to achieve great popularity over three decades in the Senate. His Republican opponent Jay Wolfe received 20% less of the vote than McCain. (Rockefeller won almost every county)
Mary Landrieu (LA), Max Baucus (MT), and Tim Johnson (SD) secured easy reelection victories. Considering Landrieu almost lost in 1996 and 2002, she was probably quite proud of her triumph over future Republican Senator John Kennedy. (Kennedy received 13% less of the vote than McCain) Landrieu dominated in Cajun Country and the River Parishes, even outrunning Obama in Orleans to keep Louisiana in the Democratic column.
Max Baucus won reelection easily, taking 73% of the vote. His Republican opponent was an actual liberal named Mike Kelleher; Baucus carried every county. Johnson had won a razor-thin victory against Congressman John Thune in 2002, indirectly leading to his colleague Tom Daschle’s 2004 loss. The GOP did not seriously challenge Johnson in 2008, and he won easily.
In North Carolina, Senator Elizabeth Dole lost reelection to a rising star state Senator named Kay Hagan. Early on in the cycle, Dole seemed like a favorite for a second term. The Tar Heel State had been partial to Republicans at the federal level, and Dole’s universally beloved status helped her hold on to Jesse Helms’s seat in a tough 2002 contest. But 2008 saw the political tides turn, making Barack Obama the first Democrat to carry the state since Jimmy Carter in 1976. Dole underperformed McCain throughout, losing handily.
Jim Inhofe, a serial underperformer in Presidential years, had a closer than expected reelection in Oklahoma against Democrat Andrew Rice; he received about 9% less of the vote than McCain, losing four counties and underperforming across Little Dixie.
Unlike Dole, who underwhelmed in loss, Oregon’s Republican Senator Gordon Smith managed to do quite well. (He only came three points away from victory, and many Republicans still blame Constitution Party nominee David Brownlow for spoiling the election) Smith received 5% more of the vote than McCain, keeping his race close against state House Speaker Jeff Merkley. Originally ahead in the polls, October saw Democrats jump into first place. Smith ran ahead of McCain throughout the state, with particularly salient wins in the Obama counties of Clackamas and Marion.
Republican Senate candidates greatly underperformed McCain against popular Democratic incumbents like Tom Harkin (IA), Dick Durbin (IL), and Jack Reed (RI). Unpopular former Governor Jim Gilmore, running in Congressman Tom Davis’s stead, could never generate much enthusiasm behind his campaign. He lost handily to Mark Warner, receiving 13% less of the vote than McCain and winning only the reddest parts of the Old Dominion.
In Georgia, Senator Saxby Chambliss underperformed McCain statewide. He held up fairly well in the Atlanta Metro’s then-Republican suburbs, but trailed McCain throughout rural Georgia. The contrast was particularly stark in the south, with Telfair County voting 57-43% McCain but 52-47% Democratic in the Senate contest. The freshman Republican later won a runoff against Jim Martin, with Democratic turnout falling through the floor.
Popular incumbents Lamar Alexander and Mike Enzi were both reelected easily in their heavily-Republican states, with each receiving a greater portion of the statewide vote than McCain. Compared to now, Obama’s 2008 performance in rural Tennessee is nothing short of spectacular. (He won Jackson County by about a point; it voted for Trump by 56 last year)

*WY, MS Specials, GA Runoff, and AR excluded*

2012
For the 2012 cycle, our correlation coefficient predictably increased. This time it reached 62%, the highest mark of its kind in modern American political history at that point. Interestingly, most of the split outcomes arose in lopsided Romney states; in some cases, like Missouri, these states were only getting redder at the federal level. But Democratic success in the Show Me State and Hoosier Country was mostly rooted in abysmal GOP candidacies.
The only Republican Senator elected in an Obama state was Dean Heller, Nevada’s interim incumbent. He had been elevated from the House the previous year after John Ensign’s career abruptly halted following a sex scandal. Heller faced Congresswoman Shelley Berkley, an opponent who trailed in most of the late-breaking polls and was generally considered inept despite a decade of political experience. Romney only underperformed Heller by 0.2%, allowing the Republican to win with one of the lowest plurality figures in recent memory – a mere 45.9%. (Nearly 10% of the total vote was cast for the Independent American Party and the None of these Candidates option)
In West Virginia, Senator Joe Manchin faced off against John Raese for the second time in two years. This time he won easily, with Raese putting his foot in his mouth enough to receive 26% less of the vote than Romney. Prior to his Senate appointment, Manchin had been an incredibly popular Governor of the Mountaineer State.
Indiana and Missouri should’ve been surefire sources of Republican victory, yet they weren’t. Candidate quality, at a time when voters still cared about it, turned out to be the GOP’s insurmountable Achilles Heel. In Hoosier Country, long-time incumbent-turned-foreign policy expert Dick Lugar lost his primary in a blow out to Treasurer Richard Mourdock after “going Washington”. Mourdock ended up receiving 10% less of the vote than Romney, losing to 2nd district Congressman Joe Donnelly. Most of his vulnerability came from his approval of fellow Senate candidate Todd Akin’s ill-conceived “legitimate rape” comment, an utterance that kept the St. Charles County Republican below 40% of the vote in his initially-auspicious challenge to Senator Claire McCaskill.
Republicans ran underwhelming campaigns in the red states of Montana and North Dakota too. Popular Democrat Jon Tester won a somewhat easy victory against beleaguered Congressman Denny Rehberg, though the Republican had scandals of his own. Next door, freshman Congressman Rick Berg lost an incredibly narrow upset to former state Attorney General Heidi Heitkamp. Incumbent Democrat Kent Conrad retired in 2012 fearing a difficult reelection, but would’ve probably had a surprisingly comfortable victory if he had run considering Heitkamp managed to hold his open seat. 
In Massachusetts and Hawaii, credible Republicans kept the Senate margins significantly closer than their Presidential toplines. Scott Brown, who won a critical 2010 special, received 9% more of the vote than Romney, a former Bay State Governor, but ultimately lost to Elizabeth Warren. Linda Lingle, the former Governor of Hawaii, got 37% of the vote and ran well ahead of Romney in her loss to Mazie Hirono. 
Democratic Senators Kirsten Gillibrand (NY), Bill Nelson (FL), and Tom Carper (DE) also did exceptionally well in their states, with Republican challengers running far behind Romney.

2016
By 2016, R-squared reached an unprecedented 76%. The linear regression had become stronger and more clearly-defined than ever before. Split partisan outcomes had all but vanished, and major differences in vote share between the Republican elements were rapidly weakening. Some would say that 2016 was the realignment before the political world knew what realignment was. The better description might focus on polarization, and its rise to prominence at unprecedented speed.
Down from six in 2012, there were no Senate races deviating from the partisan outcome at the Presidential level in 2016. As if that occurrence weren’t amazing enough on its own, vote share differences in states with similar partisan outcomes also reached all-time lows. To uncover the most interesting discrepancies of the cycle, we have to look under the hood.
Strong incumbents still made a significant difference, just not quite as notable as in years past. John McCain ran well ahead of Trump in the Maricopa County suburbs, but actually underperformed in rural parts of the state that had been represented by his opponent Ann Kirkpatrick. (McCain also significantly outran Trump among Native Americans)
In Florida, Marco Rubio did incredibly well with Hispanic and black voters. His performances in South Florida and the Tampa Bay region allowed him to outpace Trump greatly, taking 3% more of the statewide vote while besting Congressman Patrick Murphy. Republicans like Johnny Isakson (GA), Chuck Grassley (IA), John Hoeven (ND), and John Thune (SD) did similarly well in their states.
Illinois saw incumbent Republican Mark Kirk do only slightly better than Trump statewide, but most of the interesting differences between the topline and the Senate race came on the regional level. Kirk, a god of the once-GOP Illinois collar, managed to outrun Trump in counties like DuPage and Lake. Ironically, Chicagoland Congresswoman Tammy Duckworth managed to nullify his ancestral holdings by dominating in the downstate. She won the 12th Congressional district, and notably carried Gallatin County even as it backed Trump by 47 points at the top of the ticket.
In Wisconsin and Pennsylvania, Ron Johnson and Pat Toomey both won hotly-contested reelection battles that many had expected them to lose; both exceeded Trump in statewide vote share. Johnson outran Trump in the WOW counties, Toomey outran him in the Philly Collar, but both underperformed their party’s enigmatic new standard-bearer in the rural parts of their states. 
Chuck Schumer, a powerful Democratic incumbent who would become Senate Minority Leader after the election, managed to dominate in upstate New York. (He also won over Trump-heavy Orthodox Jewish communities throughout Brooklyn) Cracking 70%, Schumer carried almost every county in the state. Republican Senate retread Wendy Long finished with 9% less of the vote than Trump.
Two Republicans underperformed in otherwise GOP-oriented states. Facing a tough challenge from a moderate young Democrat named Jason Kander, Missouri Senator Roy Blunt failed to secure a majority of the vote. He underperformed Trump throughout the state, including the rural country. In Kentucky, libertarian Rand Paul took 5% less of the vote than Trump; his opponent Jim Gray even won the state’s 6th Congressional district, a seat that had evaded Democrats since Ben Chandler’s 2012 loss.
*California excluded*
2020
93% is a figure that would have been unbelievable back in 2000. It truly does represent the culmination of every malignant aspect of political polarization fueling the incessant rise of straight-ticket voting in federal races. As an analyst, the growing strength of the depicted correlation makes me somewhat disappointed. The days of interesting elections and strange outcomes are coming to an end, with voters seemingly more concerned with unanimity than contrast. Suffice to say, 2020’s differences between the Presidential and Senate levels were rather minute.
There was improvement compared to 2016 on some level. One state had a partisan split between its Senate race and the Presidential topline. That state was Maine, home to moderate Republican Susan Collins. 
Going into the election, punditry pretty much considered her time in the sun to be up. Poll after poll showed state House Speaker Sara Gideon comfortably ahead, and most of us found it difficult to see Collins surviving a lopsided Biden victory in the state. Collins’s vote for Kavanaugh in 2018 certainly added to the partisan firestorm, leading almost everyone in the political world to write her off as a victim of polarization before she could even prove herself; that was not the case. 
She received 7% more of the vote than Trump, won an outright majority to obviate the RCV system, and embarrassed Gideon beyond recognition. It was a welcome reprieve to see that the appeal of incumbency in a hostile state could still make a difference in the Trump-era.
Other Republicans managed to finish ahead of Trump by vote share too. In Colorado, Cory Gardner ended up being dead-on-arrival as expected, but still ran ahead of the President in the Denver suburbs. David Perdue, who would go on to lose his runoff in January, outran Trump in the Atlanta Metro. John James, an impressive candidate in his own right, staved off disaster in Western Michigan and ended up finishing ahead of the President statewide. Deb Fischer and Mark Ronchetti were also notable.
Most Democratic Senate candidates actually did quite well given Biden’s performances in their states. Joni Ernst underperformed Trump against Theresa Greenfield; Roger Marshall did the same while beating Barbara Bollier; Inhofe (OK), Tillis (NC), Daines (MT), and McConnell (KY) all ran behind the topline. Pundits may have expected more of someone like Montana Governor Steve Bullock or heavily-funded outsider Amy McGrath, but both actually did exceptionally well given the topline headwinds and the increasing decline of variation in Senate outcomes. (Check out Lakshya’s piece for more on this phenomenon)
*AR and GA Runoffs excluded, GA Regular Round 1 included*

Conclusion

Republican Senate vote share is more tied to Republican Presidential vote share than ever before. There is quite simply no way of overstating it. Making a more encompassing conclusion is easiest. Ticket-splitting is definitely on the decline in federal races, and there isn’t much doubt that the topline will continue to grow in importance if polarization remains the most important factor influencing American politics.

My name is Harrison Lavelle and I am a political analyst studying at the College of New Jersey. As a co-founder and partner at Split Ticket, I coordinate our House coverage. I write about a variety of electoral topics and produce political maps. Besides elections, my hobbies include music, history, language, and fitness.

Contact me at @HWLavelleMaps or harrisonwlavelle1@gmail.com

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