Republican Donald Trump has refused to commit to accepting the election result if he loses, in the final TV debate against Hillary Clinton.
"I will tell you at the time," he told moderator Chris Wallace. For days he has claimed the election is "rigged".
The Las Vegas debate continued the campaign's bitter tone, with Mr Trump calling Mrs Clinton a "nasty woman".
Polls show Mr Trump is losing in key battleground states after facing a slew of sexual assault allegations.
The final battle of wits came less than three weeks before election day on 8 November.
The candidates declined to shake hands before and after the political sparring, setting the tone for what would later become yet another debate marked by shouting and interrupting.
Mr Trump appealed to the Republican establishment by vowing to appoint Supreme Court justices with a "conservative bent" who would overturn a key ruling that made abortion legal in the US and protect gun rights.
He also stuck to his pledge to deport undocumented immigrants and secure US borders.
Meanwhile, Mrs Clinton firmly declared she would stand up for the LGBT community, defend abortion rights, focus on restoring the middle class and equal pay for women.
"The government has no business in the decisions that women make," she said
In one of the more striking moments, Mr Trump twice declined to say whether he would accept the election's outcome, breaking with the country's long-standing tradition of a losing candidate's concession after the votes are counted.
"That's horrifying," Mrs Clinton shot back.
"He is denigrating and he is talking down our democracy. And I, for one, am appalled that somebody who is the nominee of one of our two major parties would take that kind of a position."
Mr Trump's response drew sharp criticism from Republican Senator Lindsey Graham, who said the candidate was "doing the party and country a great disservice by continuing to suggest the outcome of the election is out of his hands and 'rigged' against him," according to a statement.
Nicolle Wallace, a NBC News analyst and advisor to Senator John McCain's 2008 presidential campaign, said on NBC: "He may as well have laid down in his own coffin with a hammer and nail and pounded it in himself."
Other highlights from the debate at the University of Nevada, include:
- Mrs Clinton says Mr Putin wants Mr Trump elected because he wants a puppet as US president
- "We have some bad hombres and we are going to get them out," said Mr Trump, as he stood by his pledge to build a border wall
- Mrs Clinton said she would introduce the biggest jobs programme since World War II
- Mr Trump suggested she and President Barack Obama co-ordinated the violence at his rally in Chicago earlier this year
When asked about her paid speech to a Brazilian bank in which she spoke of her dream of open trade and open borders, Mrs Clinton said she was talking about energy policy.
In one key exchange, he attacked her 30 years of "very bad experience" and she responded by going through her timeline, comparing where she was to where Trump was.
While she was in the White House helping to track down Osama Bin Laden, "he was hosting the Celebrity Apprentice", she said.
Not a good night for Trump - Anthony Zurcher, BBC News, Las Vegas
Donald Trump tried to be restrained. He really did. During the first section of the third presidential debate, when the topic was the Supreme Court, if you squinted you could almost imagine that this was just another presidential race, with two candidates squaring off and vigorously discussing their public policy positions on abortion and gun control.
Even the immigration discussion started reasonably civilly, until Mrs Clinton pivoted to turn a question about WikiLeaks into an attack on Donald Trump's relationship with Russia and Vladimir Putin.
At that point, interrupting Mr Trump returned. He called Hillary Clinton a liar and after she called him a Russian puppet, he said that was in fact her. By the time the topic turned to "fitness to be president," the stage was set for a total meltdown. He said the women who has accused him of sexual harassment were in it for the fame and Clinton campaign stooges.
Perhaps most remarkable, however, was when moderator Chris Wallace asked Mr Trump to support the election results, no matter how they came out. He refused to do so. He accused the media of poisoning people's minds. He said Mrs Clinton shouldn't have been allowed to run for president. It was Mr Trump in a bunker, settling scores and lashing out at enemies real and perceived.
At that point, the headlines were written and the debate results were locked in. It was not a good night for the Republican.
Mr Trump has faced damaging fallout after a video emerged of him making obscene comments about groping women, with senior Republicans deserting him.
When pressed about sexual assault allegations made against him by several women in the wake of the video, Mr Trump said the claims had been "largely debunked".
In addressing the groping accusations, Mrs Clinton said: "Donald thinks belittling women makes him bigger".
"Nobody has more respect for women than I do," Trump said as the crowd was heard scoffing.
He also blamed Mrs Clinton, whose campaign he said was responsible for drumming up the allegations.
