Many Trump voters back Senate Democrats, making the chamber a toss-up in the fall

But Kuriwaki found little evidence in his research that ticket-splitters are “super-strategic or wanting balance,” and Macdonald is skeptical as well, especially since no one is explicitly making a “rein in Trump” argument this year.

The other theory is that voters simply become comfortable with congressional incumbents regardless of party. When a sitting president is unpopular, voters may blame him for things they’re unhappy about — inflation, say, or the border — but they’re less likely to hold individual members of Congress responsible.

Tester and Brown are good examples of Democratic incumbents who have managed to separate themselves from Washington, according to Macdonald. Tester is a family farmer who lost several fingers in an equipment accident, and “that plays in Montana,” he said, while Brown “has the pulse of Ohio,” and both are experienced, well-funded candidates facing off against newcomers.

Tester is a populist who supports the Second Amendment and veterans’ issues, noted Christopher Muste, a political scientist at the University of Montana. Tester is “running ads against new people coming into the state and making it more expensive,” he added, and positions himself as a bipartisan centrist.

Candidate quality matters, Smith agreed. In Arizona, controversial Republican Kari Lake “is not seen as the most senatorial of candidates,” he commented, while in Wisconsin, Republican Hovde is being attacked as a carpetbagger from California.

But the problem with the incumbency theory is that, statistically, the advantage of incumbency has been declining steadily in recent years, Kuriwaki said.

“In the 1970s and ’80s, the incumbency advantage was much larger, but lately partisanship has become more important," he said.

Stephen Craig, editor of the book "Ambivalence and the Structure of Political Opinion," noted that “partisanship has been the strongest driver of vote choice” and “is an even stronger predictor of the vote today than it used to be.”

In some races, a divisive primary election — such as between a traditional Republican and a MAGA candidate — can prompt voters to split their tickets, Smith noted.

But another theory is that this year’s spike in ticket-splitting is a symptom of a larger party realignment. The high-water mark of ticket-splitting from 1968 through the early 1980s coincided with a sea-change in which the solidly Democratic South became a Republican stronghold, while Republicans started to be eclipsed in the Northeast, reflecting the parties’ altered attitudes toward civil rights and other issues. It’s possible that the current realignment — with Republicans under Trump becoming an increasingly multi-ethnic working-class party while Democrats corner the educated elite — is producing a slice of the electorate that likes Trump but hasn’t become comfortable with the Republican party as a whole.

“Voters themselves have not settled,” said Muste. He thinks many ticket-splitters are “people who are culturally conservative and economically progressive, or vice-versa,” and who no longer have a clear home — such as moderately conservative Republican women who are concerned about abortion rights.

Historically, there’s been little hard data about what kinds of voters split their tickets. One study in 2010 found that ticket-splitting was more common among white voters, as well as those with a lot of trust in government, but Kuriwaki’s more recent research found that race didn’t have a significant effect. His studies showed that ticket-splitting was most common among low-information voters, true moderates, and people whose views don’t fit neatly within the political spectrum.

Even with the potential for a lot of ticket-splitting this year, the Senate map is daunting for Democrats. Republicans currently hold 49 Senate seats, and are virtually guaranteed to pick up one more in November due to the retirement of conservative Democrat Joe Manchin in ruby-red West Virginia. If Donald Trump wins the presidency, Republicans merely need to hold onto those 50 seats to control the Senate (because the vice-president can break a tie). If Biden wins, Democrats need to hold all their current seats except West Virginia, or try to flip a Republican seat.

Republicans have no obvious weaknesses, but Democrats have taken some hope from a new Florida Atlantic University poll that shows the state’s Republican Senator Rick Scott running only two points ahead of former Democratic U.S. Representative Debbie Mucarsel-Powell, an immigrant from Ecuador — a difference that’s within the poll’s margin of error.

Scott, a businessman and former governor, won his previous Senate bid in a squeaker that was so close that it triggered a mandatory recount, but that was in 2018, which was a Democratic-leaning year. Macdonald, the Florida professor, said the recent poll may be an outlier and he'd be “absolutely stunned” if Scott lost in 2024.

On the other hand, Republicans are behind in the polls in every state currently held by Democrats (except West Virginia), so if Biden wins the election, the GOP might not be able to pick up the 51^st seat it would need to control the upper chamber.

Perhaps the Republicans’ best chance to flip a seat is in Michigan, where there is no incumbent, because Democrat Debbie Stabenow is retiring, and the state of the race is confusing because primary elections won’t be held until Aug. 6. On the Democratic side, U.S. Representative Elissa Slotkin appears to be the clear primary favorite, while among Republicans, polls show former U.S. Representative Mike Rogers, who was endorsed by Trump, in the lead but with about half the electorate undecided.

As between Slotkin and Rogers, the polling averages currently show Slotkin with a consistent lead but in the very low single digits.

With ticket-splitters favoring Democratic senators across many red and purple states, however, the Senate may well end up 50-50, with the winner of the presidential race getting the Senate as a bonus.

“If Trump wins, Republicans get the Senate,” Macdonald said. “If Biden wins, then it’s up to Tester.”

Of course, it’s possible that a number of voters will revert to their partisan habits once they get into the voting booth, as sometimes happens, warned Craig. “Don't believe everyone who says they plan to split their ticket,” he said.

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