House Speaker Paul D. Ryan dealt a hammer blow to Donald J. Trump’s presidential candidacy Monday, telling Republican lawmakers that he would no longer defend Mr. Trump and would focus instead on defending the party’s majority in Congress.
But in an illustration of Mr. Trump’s powerful grip on much of the party, Mr. Ryan faced angry blowback from conservative lawmakers supportive of Mr. Trump.
After Mr. Ryan announced his decision in a conference call Monday morning, a stream of hard-liners came on the line to urge their colleagues not to give up on Mr. Trump, and complained that Mr. Ryan was effectively conceding the presidency.
Mr. Ryan initially urged his members to focus on their own re-election campaigns and to make individual decisions about how to handle Mr. Trump, according to two people who were on the call, who spoke on condition of anonymity.
But after an uproar from his own caucus, Mr. Ryan came back on the line, about 45 minutes into the call, to reassure them.
Mr. Ryan said he would dedicate himself full time to keeping control of the House and said flatly that he “won’t defend” Mr. Trump, people on the call said. And while he did not say he was withdrawing his endorsement of Mr. Trump, some of the House members took it that way and angrily attacked him for effectively giving up on the party’s candidate.
Representative Dana Rohrabacher, a veteran California conservative, was particularly heated, according to House Republicans on the call. Representative Trent Franks of Arizona, using graphic language to describe abortion, made an ardent case for staying with Mr. Trump because his Democratic opponent, Hillary Clinton, would support a policy of destroying fetuses “limb from limb.”
AshLee Strong, a spokeswoman for Mr. Ryan, confirmed that his sole priority for the remainder of the election would be defending congressional Republicans.
“The speaker is going to spend the next month focused entirely on protecting our congressional majorities,” Ms. Strong said.
Ms. Strong said there was “no update” regarding Mr. Ryan’s endorsement of Mr. Trump.
Few anti-Trump voices spoke up on the call. Representative Martha Roby of Alabama, a defector from Mr. Trump on Saturday, said she would contribute significant funds to help Republicans hold the House majority. But she said she would speak with colleagues in private about her decision to withdraw her endorsement in the presidential race.
Representative Charlie Dent of Pennsylvania, a vocal critic of Mr. Trump, asked his colleagues if they were truly confident that there would be no more damaging disclosures about Mr. Trump. In any case, Mr. Dent argued that the presidential race was now effectively over for Mr. Trump.
Graphic: More Than 160 Republican Leaders Don’t Support Donald Trump. Here’s When They Reached Their Breaking Point.
The announcement from Mr. Ryan comes as a bitter disappointment to Mr. Trump’s campaign, which had hoped his debate performance would halt an exodus of fellow Republicans.
Mr. Trump’s candidacy was already in a dire condition before Mr. Ryan’s gesture of rejection: A poll published Monday by NBC News and The Wall Street Journal found him trailing Mrs. Clinton by a wide margin and drawing less than 40 percent of the vote. The survey was taken before Sunday night’s debate.
Early on Monday, Mr. Trump’s advisers implored members of the party to hang with their nominee, and sought to project an aura of confidence after Mr. Trump’s aggressive clash with Mrs. Clinton on Sunday night.
Kellyanne Conway, Mr. Trump’s campaign manager, said on “CBS This Morning” that she hoped Mr. Ryan “keeps his word” and maintained support for Mr. Trump.
But in a potentially ominous sign for the party, Ms. Conway also offered a note of warning for Republicans fleeing Mr. Trump: Mr. Ryan, she noted, had been booed by Trump fans over the weekend in Wisconsin after asking Mr. Trump not to attend a political event in his home state.
Ms. Conway also repeatedly indicated that she was aware of Republican lawmakers who had behaved inappropriately toward young women, and whose criticism of Mr. Trump was therefore hypocritical.
In an effort at reassurance, Gov. Mike Pence of Indiana, Mr. Trump’s running mate, denied in several television interviews that he was thinking of leaving the Republican ticket. Mr. Pence said he was fully committed to the race and would stand “shoulder to shoulder” with Mr. Trump for the duration.
“Donald Trump stepped up and won the debate last night,” Mr. Pence said on Fox News. “He showed humility and he showed strength and he expressed genuine contrition.”
Some Republicans had hoped that Mr. Pence would abandon Mr. Trump, crippling Mr. Trump’s candidacy and allowing the party to designate someone else as its standard-bearer. Several prominent Republican officials, including Senators Kelly Ayotte of New Hampshire and Rob Portman of Ohio, said over the weekend that they planned to cast protest votes for Mr. Pence instead of Mr. Trump for president.
Mr. Ryan’s huddle with House Republicans was the first of multiple war councils in Washington on Monday, as Republicans weighed how to handle a nominee whose campaign has appeared to unravel in recent days. The Republican National Committee, which has been fiercely loyal to Mr. Trump, was to hold a conference call with its members later in the afternoon.
No new prominent Republicans have withdrawn their endorsements from Mr. Trump since the end of the debate, but there was a palpable fear throughout the party that Mr. Trump had already been damaged beyond repair.
And despite his campaign’s insistent declarations of victory, Mr. Trump appeared to inflict new harm on his candidacy on Sunday night, handing new political ammunition to Mrs. Clinton, and Democrats aiming to make Republicans pay a price for supporting his campaign. He made a series of incendiary and damaging statements in the debate, in one instance declaring that Mrs. Clinton would be in jail if he were president, and another confirming that he had avoided paying federal income tax for years.
Robby Mook, Mrs. Clinton’s campaign manager, seized on Mr. Trump’s remark about jailing a political rival, and called on Mr. Trump to apologize.
“It’s chilling that Donald Trump thinks that the presidency is like some banana republic dictatorship where you can lock up your political opponents,” Mr. Mook said.
Mr. Mook also criticized Mr. Trump for what he described as a “stunt event” on Sunday evening, in which Mr. Trump brought together three women who have accused Bill Clinton of sexual misdeeds in the past for an impromptu statement to the media in St. Louis.
It appears likely that Mr. Trump and his allies will continue to make an issue of Mr. Clinton’s past conduct for the duration of the race. After having mused for months about raising Mr. Clinton’s indiscretions, Mr. Trump made them an issue on Sunday night.
But Mr. Trump’s top surrogates continued to attack Mr. Clinton on Monday morning. Ms. Conway, Mr. Trump’s campaign manager, suggested that television news shows book Mr. Clinton’s accusers for interviews.
Even Mr. Pence, who has made a practice of avoiding personal attacks for much of his political career, jabbed at Mr. Clinton on CNN for his liaison with Monica Lewinsky, whom Mr. Pence repeatedly reminded viewers was “a 23-year-old intern in the White House.”
Correction: October 10, 2016
An earlier version of this article misstated the number of women whom Donald J. Trump held a press conference with to accuse Bill Clinton of sexual misdeeds. It was three, not four. A fourth woman at the event criticized Hillary Clinton for providing legal representation for an accused child rapist.