“During the time between the election and the next Congress, perhaps as many as 20 percent of those voting in Congress will have lost their jobs, either voluntarily or involuntarily,” said Representative Mick Mulvaney of South Carolina, a Republican. “It doesn’t make sense, when the body is populated in such a fashion, to make final decisions on critical issues.”

Some House Republicans also fear a large spending bill, known as an omnibus, that would replace appropriations measures to fund the government, possibly at levels far higher than current spending. “What’s going to happen is they’re going to come back in the lame-duck session and plus up spending,” said Representative Thomas Massie, Republican of Kentucky.

Lame-duck sessions can indeed be vehicles for omnibus spending bills. However, the House Republicans who despise those bills are the same ones who have voted down individual appropriations measures or helped load them up with policy riders that contributed to their failure on the House floor. This has made omnibus and short-term broad spending bills hard to avoid, short of a government shutdown. And, notably, Mr. Ryan has repeatedly rejected the idea of an omnibus.

There is also fear that a trade bill, which has tenuous support in both chambers and which Mr. Obama greatly desires, would sail through Congress. “Both presidential candidates say they are against the trade bill,” Mr. Gosar said. “In that sense, it’s perfect timing for them to do it in lame duck.”

On Tuesday, Max Baucus, the ambassador to China and a former Democratic senator from Montana, was on Capitol Hill to make a final pitch for the trade deal, showing that there was still interest in passage this year.

It would be virtually impossible for a lame-duck Congress to approve the Pacific trade accord, the largest regional trade deal in history. The “fast-track” trade negotiating law that helped Mr. Obama complete the accord requires hurdles Congress must clear before a final up-or-down vote on any trade deal, including hearings, a public drafting of legislation, and a full debate and vote in both chambers.

For that process to conclude this year, congressional leaders would almost certainly have to hold the first hearings, at least, before Election Day. “I don’t think the predicate has been laid,” said Mr. Lott, who supports the measure. “That would be a mistake.”