“Biden really does seem to be elected by the so-called Never Trump Republicans. They are the people who came out for him,” said Michael Miller, a political science professor at Barnard College.

That helps explain how Republicans picked up nearly a dozen seats in the House, slicing the Democratic majority to the smallest margin in decades even as President Trump lost the White House.

A Fox News Voter Analysis revealed that Mr. Biden won 96% of voters who also cast their ballot for a House Democratic candidate, while Mr. Trump won 93% of voters who also voted for a House Republican.

“That gap seems small, but can be very important in close states,” Mr. Miller said. “One of the conclusions from the data here is that Joe Biden was elected by Republicans.”