In the final debate against rival Hillary Clinton, Donald Trump has refused to commit to accepting the election result if he loses.
The Republican nominee told moderator Chris Wallace: “I will tell you at the time.”
For days Donald Trump has claimed the election is “rigged”.
The Las Vegas debate on October 19 continued the campaign’s bitter tone, with Donald Trump calling Hillary Clinton a “nasty woman”.
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.
Donald 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.
The billionaire also stuck to his pledge to deport undocumented immigrants and secure US borders.
Meanwhile, Hillary 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, Donald 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,” Hillary 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.”
Donald 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.
When asked about her paid speech to a Brazilian bank in which she spoke of her dream of open trade and open borders, Hillary Clinton said she was talking about energy policy.
In one key exchange, Donald Trump attacked Hillary Clinton’s 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”, Hillary Clinton said.
Donald Trump has faced damaging fallout after a video emerged of him making inappropriate comments about groping women, with senior Republicans deserting him.
When pressed about assault allegations made against him by several women in the wake of the video, Donald Trump said the claims had been “largely debunked”.
In addressing the groping accusations, Hillary Clinton said: “Donald thinks belittling women makes him bigger.”
“Nobody has more respect for women than I do,” Donald Trump said as the crowd was heard scoffing.
Donald Trump also blamed Hillary Clinton, whose campaign he said was responsible for drumming up the allegations.