Representative Ryan A. Costello, a Pennsylvania Republican not running for re-election, acknowledged the political threat to his House colleagues in a tweet after Justice Anthony M. Kennedy announced his retirement. A Supreme Court appointment, he said, “means abortion politics front & center. Not ideal for GOP branding in suburban swing House districts.”

And it is not just the court where the House and Senate approaches diverge. Senate Democrats in Republican-leaning states generally must be careful not to make the president much of an issue. Democrats vying against House Republicans in suburban communities often make Mr. Trump the issue.

Traditionally, the party in the White House suffers midterm losses in the House and Senate. The extent of the losses can vary, and the numbers are generally bigger in the House, with its larger membership and the more tentative connections between its members and a lightly engaged public.

But the trend is usually all in one direction.

Think 2006, when Republicans lost 30 seats in the House and six in the Senate during the second midterm of George W. Bush’s presidency, or 2010, when Democrats lost 63 seats in the House and six in the Senate during the first midterm under President Barack Obama.

But because of this year’s unusual midterm trajectories, it is conceivable that the final results could be as different as Democrats seizing control of the House while losing ground in the Senate.

“I think that is a very real scenario,” said Nathan Gonzales, the editor of the nonpartisan Inside Elections newsletter. “The Senate map and the House map are completely different, and the path to victory for Democrats is completely different depending on which chamber you are talking about.”

Senate Democrats are the victims of their own success. Strong election showings in 2006 and 2012 expanded their membership and extended their reach into Republican-leaning states like Montana. As a result they have 26 seats on the ballot in November, compared with just nine for Republicans. While the 10 Trump-state Democrats sweat out the election, just one Republican, Senator Dean Heller of Nevada, is running in a state carried by Hillary Clinton.