By the time Senator Mitch McConnell, the majority leader, created the final of his repeated pleas to President Trump to maintain his distance from the Senate candidacy of Roy S. Moore, it was also late.

To Mr. McConnell, only the president could extinguish a fire that he sees as endangering Republicans’ Senate majority. But Mr. Trump, speaking by telephone last Tuesday with Mr. McConnell, responded with the exact same argument he had been creating for days inside the White Residence.

The women who have referred to as Mr. Moore a sexual predator, the president believes, could not be telling the truth.

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“Forty years is a long time. He’s run eight races, and this has in no way come up,” Mr. Trump stated to the television cameras on the South Lawn hours after his conversation with Mr. McConnell, efficiently endorsing Mr. Moore just before boarding Marine 1. “He says it didn’t take place,” the president added. “You have to listen to him, also.”

Mr. Trump’s selection to reject each extended-shot strategy to save the Senate seat reflects the crucial that an unpopular president faces to retain his political base, a determination that he should stick to his own instincts following obtaining felt steered into a disastrous earlier endorsement in the Alabama race, and even his insistence that he himself has been the victim of false accusations of sexual misconduct.

But in tying himself to Mr. Moore even as congressional leaders have abandoned the candidate en masse, the president has reignited hostilities with his personal party just as Senate Republicans are rushing to pass a politically essential tax overhaul. Mr. McConnell and his allies have been specifically infuriated as Mr. Trump has reacted with indifference to a series of concepts they have floated to attempt to block Mr. Moore.

The accusations against Mr. Moore have lifted Democrats’ hopes of notching a uncommon victory in the Deep South in subsequent month’s special election, which would narrow the Republican Senate majority to a single seat. Just as significantly, the president has handed the Democrats a political weapon with which to batter Republicans going into the midterm elections: that they tolerate child predation.

“I was shocked, and I consider it’s a higher-risk move,” mentioned Senator Lindsey Graham of South Carolina, who has established a rapport with Mr. Trump.

As Mr. Moore has rejected calls to drop out even as much more females have accused him of preying on them when they had been teenagers, Republicans have offered up any hope that he will fold his campaign. Mr. Trump has repeatedly told his aides that he does not think Mr. Moore would ever quit.

What the president did not foresee was that the friction would reach inside his immediate loved ones. He vented his annoyance when his daughter Ivanka castigated Mr. Moore by saying there was “a unique place in hell for people who prey on youngsters,” according to 3 staff members who heard his comments.

“Do you believe this?” Mr. Trump asked a number of aides in the Oval Workplace. Mr. Moore’s Democratic opponent in the Alabama race, Doug Jones, speedily turned her comments into a campaign ad.

But anything deeper has been consuming Mr. Trump. He sees the calls for Mr. Moore to step aside as a version of the response to the now-renowned “Access Hollywood” tape, in which he boasted about grabbing women’s genitalia, and the flood of groping accusations against him that followed soon after. He suggested to a senator earlier this year that it was not authentic, and repeated that claim to an adviser a lot more not too long ago. (In the hours soon after it was revealed in October 2016, Mr. Trump acknowledged that the voice was his, and he apologized.)

So Mr. Trump has been specifically open to the notion, pushed by Mr. Moore’s defenders, that the candidate is becoming wrongly accused, even as Mr. McConnell and a parade of other Republicans have stated they think the accusers. When a group of senators gathered with the president in the White House final week to go over the tax overhaul, it took little to get Mr. Trump onto the topic of Mr. Moore — and he right away offered up the very same it-was-40-years-ago defense, according to officials at the meeting.

Mr. Trump’s responses to the Moore revelations have been pronounced but not constant. He accepted the candidate’s initial denials, and then was shocked at how tepid Mr. Moore appeared when asked during an interview with Sean Hannity whether or not he nevertheless maintained his innocence, according to one individual close to the president.

Privately, Mr. Trump has acknowledged that he is producing a cold political calculus in the hope that the Republicans will hold on to the seat. A White Property official on Saturday reiterated the president’s view that he believes Mr. Moore must quit the race if the allegations are proved accurate, but the official stressed that the candidate has denied them.

Absent action from Mr. Trump, celebration leaders have explored — and abandoned — a number of techniques to derail Mr. Moore. They deemed recruiting yet 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 accomplishment.

Each polls, commissioned by Republican groups in mid-November, located Mr. Jones top Mr. Moore in a head-to-head election and winning handily in a three-way race, according to individuals 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 shed, 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 celebration into an agonizing monthslong debate more than no matter 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 individuals who have spoken with him — his starkest acknowledgment so far that the political atmosphere 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, even though Mr. McConnell has made clear that he thinks that the candidate is unlikely to win.

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

“As much as individuals would like to assume that, as Louis XIV said, ‘I am the state,’ there is much more than a single individual who represents the Republican Party, and the preponderance of the celebration has dissociated itself from Moore,” mentioned 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 ought to be illegal — effectively before getting accused of creating sexual advances on minors when he was a district lawyer in his 30s.

Mr. McConnell even enlisted Washington campaign lawyers with knowledge in Alabama elections to devise a 4-web page memo outlining a legal avenue to block Mr. Moore’s path, but the White House counsel’s office ignored the document completely. “All you can do is determine a way out of the mess, and if individuals don’t want to adhere to it, that’s on them,” stated Josh Holmes, 1 of Mr. McConnell’s closest political advisers.

Mr. McConnell and his lieutenants deemed a write-in candidacy and identified the prospect of wooing Lawyer Basic Jeff Sessions, whose old seat is in play, to be particularly appealing.

And Senate Republicans initially seemed to have allies in the West Wing: Mr. McConnell discovered Mr. Trump and Vice President Mike Pence to be receptive when he 1st talked to them about disavowing Mr. Moore.

But even just before Mr. Trump returned to American soil, he was becoming uneasy about producing such a break

Flying back from Asia, Mr. Trump was told by aides that the Republican National Committee was handling the Moore circumstance. Some of Mr. Trump’s advisers insisted that the move to reduce off funds was smart, but by the time he returned to the White Property, he had heard from others who thought the selection was misguided, according to two officials close to the president.

Quickly soon after, the president’s former chief strategist, Stephen K. Bannon, started aggressively urging Mr. Trump not to break with Mr. Moore, arguing that he need to not get crosswise with his voter base, even though it was not clear if the two guys spoke directly. The president is nevertheless smarting over his choice to fly into Alabama in September on behalf of Senator Luther Strange, the appointee holding the seat, only to see Mr. Strange drop a runoff by more than nine points.

Mr. Pence, after initially issuing a statement critical of Mr. Moore, has since followed the president’s lead. At a recent meeting of the Republican Governors Association, Mr. Pence spoke privately with Gov. Kay Ivey of Alabama, according to Republican officials familiar with the conversation.

The vice president inquired about the governor’s view of the matter, but did not press Ms. Ivey on Mr. Moore. She signaled no willingness to intervene, only reiterating to Mr. Pence that Alabamians would render their judgment and that the election take location subsequent month as scheduled. (When Senator Richard C. Shelby, Alabama’s most senior lawmaker, referred to as Ms. Ivey, she barely let him commence speaking before bluntly informing him that she would not change the date.)

Mr. Trump has not spoken with Ms. Ivey, which has produced Mr. McConnell’s final Hail Mary a nonstarter.

The campaign lawyers commissioned by the Senate leader last week sent a memo to the White Home counsel, Donald F. McGahn II, arguing that, based on Alabama precedent, if Mr. Strange had been to resign, Ms. Ivey could appoint a new senator. They also created the case that Ms. Ivey was within her rights to delay the unique election.

“Our recommendation is to combine Actions 1 and two: Strange resigns the governor fills the vacancy with a new appointee and the governor delays the particular election to give the new appointee time to run as an independent candidate,” the lawyers wrote.

Ought to Mr. Jones win, Democrats would want to take only two a lot more seats in 2018 to regain a majority in the Senate — nevertheless a hard job, but 1 practically unimaginable just a month ago. A victory for Mr. Moore could be just as punishing for Republicans, since it could taint their candidates across the country by association with a man accused of child molestation.

Scott Jennings, a Republican strategist close to Mr. McConnell, said the race had created into a no-win situation.

“Either we’re saddled with a Democrat in a seat that ought to be Republican,” Mr. Jennings mentioned, “or we’re saddled with a brand anvil that is going to drag down the president, drag down the Senate, drag down the party and plunge the Senate into instant turmoil when he gets there.”

For its component, Mr. Moore’s campaign is thrilled to have the president’s tacit help and is promising to highlight it.

“We’re going to make it clear to the voters of Alabama that Roy Moore is the candidate to assist President Trump get a conservative Supreme Court and reduce taxes,” mentioned Brett Doster, a top Moore adviser. “That will be included in our ads, absolutely.”