How Democrats won Senate seats in states that Trump carried

As the dust begins to settle on the 2024 election, one eye-opening result has been the number of split-ticket outcomes in states between their presidential and U.S. Senate races. Across the 2016 and 2020 presidential elections, just one state — Maine in 2020 — didn't vote for the same party for both offices when both were on the ballot at the same time. But 2024 appears set to have at least three split-ticket outcomes, and most likely four as things stand.

At the time of writing, ABC News has projected split-ticket outcomes in Michigan, Nevada and Wisconsin — all states President-elect Donald Trump carried while the Democratic nominee for Senate won. One other state that Trump won — Arizona — also looks more likely than not to have a split-ticket outcome, as Democratic Rep. Ruben Gallego leads by a bit more than 2 percentage points with 91 percent of the expected vote reporting. Conversely, Pennsylvania appears likely to end up as a straight-ticket Republican win for both offices, although Republican Senate candidate Dave McCormick's lead over Democratic Sen. Bob Casey Jr. — less than 1 point — may prove close enough to precipitate a recount.

All told, then, the 2024 election represents a notable uptick in split-ticket results and downturn in same-party outcomes. Based on the results as they stand right now, different parties won the presidential and Senate contests in 12 percent of the states that had both contests on the ballot, the highest share since 18 percent of 2012's presidential-Senate races had split-ticket outcomes.

Naturally, this has led to easy headlines about split-ticket voting making a comeback. And there's some truth to that, both in split-ticket outcomes and in relatively larger differences in the vote margins between presidential and Senate races in the same state. If we look at contemporaneous presidential and Senate races in which both a Democrat and Republican were on the ballot (including independent Sens. Angus King of Maine and Bernie Sanders of Vermont as Democrats), 2020 saw the narrowest gap in the margin of victory between the two major parties in these types of races since at least 1992 — 2.8 points in margin. Based on the present results, the median gap in 2024 will be higher, around 4 points — though still historically quite low, roughly half the almost 8-point mark in 2016.

But despite the indisputable impact of this year's split-ticket outcomes, that shouldn't be mistaken as evidence that there were substantially more split-ticket voters who cast a ballot for a Democrat in one contest and a Republican in the other. That's because the margins between the two major parties in states that had both offices on the ballot were very similar in each race. In fact, even as the median difference in margin between the two types of races increased somewhat from 2020, the correlation between the margins in the presidential and Senate races was about 0.95 (out of 1, which would signify perfect similarity), essentially identical to 2020 — which was the highest in a presidential cycle since popular elections for Senate began after the ratification of the 17th Amendment in 1913.

Fact is, an exceptionally large number of close races happened to produce split-ticket outcomes in this election, even if most voters still voted for the same party for both offices. At least four of the five closest Senate races will be decided by fewer than 2 points in margin — depending on Arizona's final count — at least tying this cycle with 2020 for the most Senate races decided by such slim margins in a presidential year since at least 1992. And the 2020 case was unusual in that two of those races were Georgia's double-barrel Senate contests that both went to January 2021 runoffs. This time around, four of these very close contests are on course to produce split-ticket results, each to the advantage of Democrats. Many factors may have played into Democratic Senate contenders narrowly outperforming Harris and Republican candidates slightly underperforming Trump, including incumbency and candidate quality.

Yet only one of the four likely split-ticket outcomes appears to have come about because a big percentage of voters cast ballots simultaneously for Trump and the Democratic Senate nominee. In Arizona, which remains unprojected, Gallego has won nearly 6 percent more raw votes than Harris has, while Republican Kari Lake has won almost 10 percent fewer raw votes than Trump — a signal that a not-insignificant group of Trump voters backed Gallego.

But in other states with split-ticket outcomes, a comparison between the presidential and Senate votes suggests that at least a fair number of Trump voters failed to back Republican Senate candidates — as opposed to backing Democrats. Now, some ballot roll-off is to be expected, as there's a longstanding pattern whereby some voters choose not to vote for lower offices (also known as undervoting). But this, in combination with third-party voting, created notable GOP Senate shortfalls.

In Nevada, Republican Sam Brown has won nearly 10 percent fewer votes than Trump, with some Trump voters certainly choosing the state's unique "None of these candidates" option. Less dramatically, Republican Mike Rogers has won 4 percent fewer raw votes than Trump in Michigan, and Republican Eric Hovde has won 3 percent fewer than Trump in Wisconsin. By comparison, the Democrats in these contests (Rep. Elissa Slotkin and Sen. Tammy Baldwin, respectively) have won between a few hundred more votes than Harris and about 1 percent fewer than Harris.

Now, a few other states did see sizable amounts of split-ticket voting between the presidential and Senate races that, potentially, stood to affect control of the Senate. In Montana, Democratic Sen. Jon Tester significantly outperformed Harris, winning 19 percent more raw votes than she did, while in Ohio, Democratic Sen. Sherrod Brown won about 5 percent more. In Maryland, former Gov. Larry Hogan gave Republicans an unusually strong candidate in a blue state, and he won a whopping 24 percent more raw votes than Trump did.

However, all three candidates failed to garner enough split-ticket votes to overcome the respective leanings of their states — even as Hogan and Tester managed to achieve two of the largest gaps between presidential and Senate races dating back to 2012.

Tester's situation is especially striking because he'd managed to overcome Montana's red lean in a presidential year before. In 2012, Tester won reelection by about 4 points even as Mitt Romney carried Montana by about 14 points in the presidential race, producing a bit less than an 18-point gap between the two contests' margins. But in 2024, Tester would have needed an even larger chasm between the Senate and presidential contests, as the state backed Trump by 20 points. Tester ultimately lost by 7.5 points, about a 13-point gap.

Overall, split-ticket outcomes absolutely impacted the 2024 race for the Senate. Had Republicans carried every Senate contest in a state that Trump won, they would be standing at 57 seats instead of what appears likely to be 53. However, the votes in presidential and Senate races are about as closely aligned as they've ever been. Only in the very closest races did the small share of split-ticket voters affect the outcomes — but in many of those contests, the voters who chose to vote for a third party or abstain from voting downballot may have had as big or an even larger effect than those who voted for a Republican for president and a Democrat for Senate.