By Stephen Collinson
(CNN) -- Donald Trump on Wednesday refused to say he would accept the result of next month's presidential election.
"I will look at it at the time," Trump said when challenged during the final presidential debate on his claims that the election is "rigged" against him.
He added: "I will keep you in suspense."
The comments at the Las Vegas showdown marked an extraordinary departure from one of the most fundamental principles of American democracy: the peaceful transfer of power after an election. They exposed a divide with Republican vice presidential nominee Mike Pence, who told CNN's Wolf Blitzer before the debate "we'll certainly accept the outcome of this election."
Trump's campaign manager, Kellyanne Conway, told CNN's Dana Bash after the debate that Trump will "accept the results of the election because he's going to win the election."
Trump's refusal to commit to accepting the outcome of the election could become a defining moment of the campaign. It was a dramatic departure from the first hour of the debate, where Trump showed unusual restraint as he and Hillary Clinton engaged in substantial policy discussions over issues such as the Supreme Court, the Second Amendment and gun rights.
Clinton, the Democratic nominee who is leading Trump in most polls, said the Republican nominee's remarks were "horrifying."
"You are not up to doing the job," Clinton charged, claiming that every time events do not turn out in his favor, Trump claimed things were "rigged" against him. She highlighted Trump's past comments lamenting results of the Iowa caucuses, legal judgments against him -- even Trump's complaints when he did not win an Emmy.
Trump's comments on the election outcome could doom his chance to win over any remaining undecided voters. The Republican nominee, down eight points in the latest CNN Poll of Polls, is almost out of time to launch what would be one of the most remarkable comebacks of modern times. A new edition of the CNN Electoral Map on Wednesday moved two key swing states, Florida and Nevada, to "lean Democrat." Two states that have voted almost exclusively Republican for decades, Utah and Arizona, are now considered battlegrounds.
Losing his cool
Trump seemed to lose his cool as the debate went on, harshly criticizing Clinton and occasionally getting testy with the debate moderator, Chris Wallace of Fox News. The debate began to take a turn when Trump and Clinton clashed over the Republican nominee's relationship with Russian President Vladimir Putin.
Clinton blasted Trump as a "puppet" of Putin and directly called on him to condemn what she said was a Russian effort to use cyber attacks to influence the election in her opponent's favor.
Trump replied that Putin had no respect for Clinton or President Barack Obama.
"That's because he would rather have a puppet as President of the United States," Clinton said, implying that Putin wanted Trump to win the election.
"No puppet. You are the puppet," Trump said.
Trump said he had never met Putin but allowed that the Russian leader had said nice things about him, and said it would be good if Washington and Moscow worked together to fight ISIS.
But he added: "This is not my best friend."
Clinton and Trump also bitterly sparred over the theme of who is qualified to be President. Wallace pressed Trump on why so many women had come forward to accuse him of sexual assault if the allegations were not true.
Trump said the claims had been "largely debunked."
"I think they want either fame or her campaign did it," Trump said, referring to the women that came forward after he said at the last debate he had never been abusive to any women.
Clinton noted that Trump had implied at several rallies that he could not have made inappropriate advances to the women because they were not sufficiently attractive.
Trump wrongly denied that he had ever made such a remark.
"Donald thinks belittling women makes him bigger. He goes after their dignity and their self-worth," Clinton said.
Treatment of women
Clinton said that Trump's treatment of women was part of pattern of behavior that saw him insult a disabled reporter, go after the parents of a fallen Muslim soldier and an American judge of Mexican descent.
She said such tactics were in line with a divisive and very "dangerous vision of our country."
The tone of the debate -- unusually substantial at the start -- never recovered once the atmosphere became charged. As the event wound down, Clinton said that under her economic plan, her and Trump's payroll taxes would go up to ensure the solvency of Social Security -- unless her rival could figure out a way to avoid paying taxes.
"Such a nasty woman," Trump interjected.
CNN's Jim Acosta, Jeff Zeleny and Dan Merica contributed to this report