Indeed, Ryan said he would devote his energy to ensuring Clinton doesn't get a "blank check" as president with a Democratic-controlled Congress, according to people on his private conference call with GOP House members. While the Wisconsin Republican did not formally rescind his own tepid endorsement of Trump, he told lawmakers they were free to do just that and fight for their own re-election.
Locals react to second presidential debate
Trump fired back on Twitter, saying Ryan "should spend more time on balancing the budget, jobs and illegal immigration and not waste his time on fighting Republican nominee."
Ryan's announcement underscored the perilous predicament Republicans find themselves in one month from Election Day. Recent revelations of Trump's predatory sexual comments about women deepened the worries among GOP officials who fear he'll drag down their own electoral prospects in November. But others look at Trump's loyal bands of supporters and see no way for Republicans in other races to win without their support.
Trump himself made no reference to Ryan and the GOP defections at a Pennsylvania rally, except perhaps one line that could apply to fleeing Republicans as well as the Democrats.
"The last 72 hours has framed what this election is all about. It's about the American people fighting back against corrupt politicians who don't care about anything except for staying in power," he said.
Trump's candidacy long ago laid bare the turmoil roiling the GOP. Some party leaders had hoped to push off a reckoning until after the election, but with Ryan and other lawmakers publicly distancing themselves from Trump - and in some cases even calling for the real estate mogul to drop out of the race - that now appears impossible.
For Ryan, the most pressing goal through the next four weeks is preventing Republicans from losing control of the House, a scenario that seemed remote as recently as a week ago. Although Republicans are not yet panicking given their wide 246-186 seat majority, Ryan and Greg Walden, the chairman of the National Republican Congressional Committee, both acknowledged on Monday's conference call that the majority was more in peril in light of Trump's problems.
Walden told lawmakers they still could win their seats, but that it would require delicate maneuvering akin to landing an airplane in a hurricane in a fog, several participants said.
On the other side of the Capitol, there were also signs that more Republican Senate candidates were moving to distance themselves from Trump. Two Republicans said they expected to see ads urging voters to back GOP Senate candidates as a check on Clinton's power in the White House, with one of the Republicans saying the spots could come as soon as this week.
Senate Majority Leader Mitch McConnell refused to even acknowledge Trump Monday, telling a group of business leaders in his home state of Kentucky that if they expected to hear him discuss the presidential race they "might as well go ahead and leave."
Clinton's campaign hammered Republicans for recoiling from Trump at this late date and urged voters to hold GOP candidates accountable for standing by their nominee for months.
"Donald Trump didn't become the nominee of his party on his own," said Jennifer Palmieri, Clinton's communications director.
Seeking to pad her lead in battleground states, Clinton was making a direct appeal for moderate Republican voters turned off by Trump. The Clinton campaign released new ads featuring Republican voters crossing party lines to cast their ballots for the former secretary of state.
"I don't always agree with her but she's reasonable and she's smart," Republican Jennifer Kohn says in one spot.
Trump supporters are furious at the notion that Republican leaders are abandoning the nominee selected by their party. In the conference call with Ryan, California Rep. Dana Rohrabacher called GOP leaders cowards," according to one participant, who like others, insisted on anonymity in order to describe the private discussions.
Corey Stewart, the chairman of Trump's Virginia campaign, said Ryan and other party elite didn't understand that "the grass roots took control of this party when we nominated Donald Trump." Stewart said he was leading a group of Republicans to protest outside party headquarters near the Capitol Monday afternoon.
Stewart was fired immediately after the protest, which drew dozens of angry Trump supporters to the front step of the Republican National Committee headquarters. Said Deputy Campaign Manager Dave Bossie: "Corey made this decision when he staged a stunt in front of the RNC without the knowledge or the approval of the Trump campaign."
Republican National Committee Chairman Reince Priebus was convening a call with party members later Monday.
Trump had hoped to stop the exodus of Republicans running away from his campaign with a solid performance in Sunday's presidential debate. He did energize his core supporters by hurling insults at Clinton - he called her the "devil" and promised to put her in jail if he's president - but he appeared to do little to win over new voters. He insisted his aggressively vulgar remarks on the videotape were mere "locker room" talk and tried to turn the attention to Bill Clinton's extramarital affairs.
AP writers Bradley Klapper, Lisa Lerer, Steve Peoples, Matthew Barakat and Adam Beam contributed to this report.