ST. LOUIS — Following perhaps the most negative debate in U.S. political history, Donald Trump and his campaign looked Monday to stop Republican defections from their ranks, while Hillary Clinton sought to keep her leads in pre-election polls.

Trump still faces challenges on the unity front: House Speaker Paul Ryan, R-Wis., told a group of congressional colleagues on a conference call Monday that he would not campaign with Trump and that House Republicans should do what they need to in order to protect their seats and the House GOP majority in the Nov. 8 elections.

"The speaker is going to spend the next month focused entirely on protecting our congressional majorities," said Ryan spokesperson AshLee Strong.

During Sunday's debate at Washington University in St. Louis, Trump expressed regret for lewd comments he has made about women and tried to counter-attack by making allegations against his rival and her husband, former president Bill Clinton — and even suggested Hillary Clinton should be in jail.

"He stepped up," Trump running mate Mike Pence told Fox & Friends early Monday, just days after he had criticized the New York businessman for his comments about women. "He showed humility — he showed strength."

Clinton told reporters after the debate that Trump engaged in an "avalanche of falsehoods," and "I really find it almost unimaginable that someone can stand and just tell a falsehood after a falsehood."

Trump entered the debate under heavy pressure over the release of a 2005 tape in which he is heard bragging about grabbing, groping, and suddenly kissing women, saying they are receptive because he is a celebrity. Dozens of Republicans called on their nominee to exit the presidential race, fearing more revelations are on the way.

During the debate at Washington University in St. Louis, the GOP nominee apologized for the video comments while dismissing them as nothing more than "locker room talk." He sought to change the narrative by extending debate invitations to women who have made sexual allegations against Bill Clinton and claimed that Hillary Clinton tried to silence them.

"Bill Clinton was abusive to women," Trump said during the debate. "Hillary Clinton attacked those same women and attacked them viciously."

The New York businessman also said that, if elected, he would appoint a special prosecutor to investigate Clinton over her private emails and Clinton Foundation fundraising.

Clinton aides described Trump's aggression as a sign of implosion.

"Trump is desperate," said Clinton campaign chairman John Podesta. "He’s trying to take this race to a place in the gutter, and we’re not going there."

Clinton spokesman Brian Fallon tweeted: "So if Trump wins, he will put his opponent in jail ... If he loses the system is rigged ... Either way, he is a threat to democracy."

Aaron Kall, director of debate at the University of Michigan, said Clinton "took no risks" during Sunday's session, and is "looking to run out the clock" as she leads Trump in the polls, both nationally and in several key battleground states.

Trump, meanwhile, played to the most conservative wing of the Republican Party, Kall said, but it is uncertain whether his approach will appeal to undecided voters.

"Trump put a finger in the dyke tonight," Kall said, "but it could still burst before election day.

Several Republicans said Trump did well enough to stop Republicans from calling on him to step aside — barring further revelations.

"I saw many Republican who don't support Trump admitting he got the job done tonight," said GOP consultant Bruce Haynes, founding partner of Washington-based Purple Strategies. "He re-established his viability. And he moves on."

GOP pollster Frank Luntz, who conducted a focus group during the debate, said the Republican candidate is back because voters "care more about Hillary's emails than Donald Trump's language in private."

Sarah Isgur Flores, a Republican strategist, said Trump "may have stopped the bleeding," but he lost a lot of blood over the weekend and needs to capture more voters in key states.

Nothing he said during the debate "changed the map the way he needed," she said.