In that election, Mr. Trump seemed at times as if he could threaten the Republican majority if he ended up losing badly. In the end, the G.O.P. lost only six net seats, while Mr. Trump ran ahead of only 26 victorious House Republicans. In some instances, the ticket splitting was substantial. There were 37 Republican-won House districts where either Mr. Trump ran at least five points behind Mitt Romney’s 2012 vote share, Hillary Clinton ran at least five points ahead of Barack Obama’s 2012 share, or both. On average, House Republicans ran 15 points ahead of Mr. Trump’s margin in these districts in 2016. Even though these districts weren’t crazy about Mr. Trump, House Republican candidates did well in them.

Democrats would go on to win 20 of those 37 seats in the 2018 midterm. Why such a backlash? Perhaps because some of these ticket splitters didn’t get the outcome they expected in 2016.

The political scientist Robert Erikson has found that in post-World War II presidential elections, some voters may split their tickets against the party they believe will win the White House as a way to put a check on the likely winner, as opposed to waiting for the midterm two years later, when the president’s party customarily struggles. The public, along with analysts and betting markets, all saw Mrs. Clinton as a favorite in 2016. “Plausibly, many who thought Hillary Clinton would win voted Republican for Congress to block, thus accounting for the Democrats’ surprisingly feeble performance at the congressional level in 2016,” Mr. Erikson wrote in the lead-up to the 2018 election.

Analysts generally do not see either party as strongly favored to win the presidency this fall, but the public seems to differ. A recent Monmouth University poll showed that two-thirds of American voters think the president definitely or probably will win. A Sanders nomination might make the public even likelier to view Mr. Trump as a favorite; Democrats, already pessimistic, are starting to hear skepticism from their own leaders about Mr. Sanders’s chances. Oddsmakers have also recently made Mr. Trump a more significant favorite.

A consequence of Mr. Trump’s chronically low approval ratings is that even if Americans ultimately decide he’s the lesser of two evils this fall, there may be some voters who back him only tepidly or anticipate his victory and don’t want his party to have total control of the government. Mr. Sanders being a potentially weak opponent doesn’t necessarily make the president a beloved incumbent.

That may work to the benefit of House Democrats, even as they now panic — rightly or wrongly — over the possibility of sharing the ballot with Mr. Sanders.