These voters are very important for the battle for the House. Democrats need to pick up 24 House seats to get a majority, and Republicans hold 25 seats in areas that Hillary Clinton carried. Mitt Romney won the districts of 13 of those seats in 2012, and his margin of defeat was smaller than Mr. Trump’s in another nine. Democrats simply cannot retake the House unless they get a lot of these voters to stick with them when Mr. Trump isn’t personally on the ballot.

“Obama-Trump” voters are the people you’ve heard a lot about recently: largely white, less educated and middle or working class. By and large the latest Voter Survey shows that they still like Trump: 76 percent approve of his performance. But like Romney-Clinton voters, they aren’t yet completely sold on their new party’s congressional candidates. While 41 percent say they will vote Republican in the fall, 44 percent say they are either unsure whom they will back or plan to vote for a third-party candidate. That’s a lot of Trump backers who haven’t yet made the leap to the G.O.P.

These voters are also important to the House battle. Republicans hold 12 House seats that flipped from Obama to Trump. They are also seriously contesting five other open seats currently held by Democrats that also flipped. Hold onto some of these and make a few pickups and Republicans can offset some losses and minimize Democratic gains.

Both types of voters are also in play in the key Senate races. Democrats represent five seats under attack this fall where Mr. Trump won by 18 percent or more. In each state Mr. Trump’s margin exceeded Mr. Romney’s by between 6 and 16 points. That’s the Obama-Trump voter at work, and Democratic incumbents need to win the bulk of these voters back to have a shot. But in three other key states — Nevada, Florida and Arizona — the balance between the two types of swing voters is more even. Lesser-educated white areas swung to Mr. Trump, while more educated areas swung to Mrs. Clinton. The candidate here who can combine appeals to both groups simultaneously is likely to prevail.

These dynamics will be on clear display in Tuesday’s special election in Ohio’s 12th Congressional District. The seat is split between an affluent, educated core that has loads of Romney-Clinton voters (Delaware and Franklin Counties) and five small-town and rural counties that have lots of Obama-Trump voters. Mrs. Clinton, for example, carried Franklin — home to the state capital, Columbus — by nearly 16 percent more than Mr. Obama did in 2012. But Trump carried each of the rural counties by between 15 and 29 percent more than Mr. Romney did.