Absent action from Mr. Trump, party leaders have explored — and abandoned — a number of ways to derail Mr. Moore. They considered recruiting another Republican to run a write-in campaign against Mr. Moore and Mr. Jones, but two private polls showed that such a candidacy would have no chance of success.

Both polls, commissioned by Republican groups in mid-November, found Mr. Jones leading Mr. Moore in a head-to-head election and winning handily in a three-way race, according to people who reviewed the results. Public polls have indicated a very close race.

Mr. McConnell and his allies have believed for weeks that disaster awaits, win or lose, if Mr. Moore remains in the race: Either the Democrats will claim the seat on Dec. 12, or Mr. Moore will win and thrust the party into an agonizing monthslong debate over whether to expel him.

The Senate leader has told fellow Republicans in private that Mr. Moore’s nomination has endangered the party’s hold on the Senate, according to people who have spoken with him — his starkest acknowledgment so far that the political environment has turned sharply against his party since Mr. Trump’s election. Mr. McConnell has also reiterated his intention to move against Mr. Moore if he is elected, though Mr. McConnell has made clear that he thinks that the candidate is unlikely to win.

Otherwise loyal Senate Republicans have started putting some distance between themselves and the president, a breach that could grow wider in the event of expulsion proceedings.

“As much as people would like to assume that, as Louis XIV said, ‘I am the state,’ there is more than one person who represents the Republican Party, and the preponderance of the party has dissociated itself from Moore,” said Senator Bill Cassidy of Louisiana.

The president blindsided congressional Republicans with his defense of Mr. Moore, who was a polarizing figure — he has said homosexual conduct should be illegal — well before being accused of making sexual advances on minors when he was a district attorney in his 30s.