In a break with decades of tradition in American presidential contests, Donald Trump refused to say whether he would respect the election result if he loses and continued to allege massive irregularities in the voter rolls during his final showdown with Hillary Clinton.
Asked if he would honour the outcome of the vote on Nov. 8, Mr. Trump said: "I will tell you at the time. I will keep you in suspense."
In a last-ditch effort to rally his supporters and deter his opponents, Mr. Trump repeatedly called Ms. Clinton a liar and denounced her actions as criminal as he sought to prevent the presidential race from slipping out of his grasp.
With less than three weeks remaining until Election Day, polls show that Mr. Trump, the Republican nominee, has fallen behind Ms. Clinton, a Democrat.
At least eight women have come forward to accuse Mr. Trump of kissing or groping them without their consent. During the debate, Mr. Trump called those allegations "fiction" and claimed they were the work of Ms. Clinton's campaign. "I didn't even apologize to my wife who's sitting right here because I didn't do anything," he said. The women who came forward were seeking fame, he continued.
Ms. Clinton had a forceful response. "Donald thinks belittling women makes him bigger. He goes after their dignity, their self-worth," she said. "That's who Donald is. I think it's really up to all of us to demonstrate who we are and who our country is."
The face-off represented a last opportunity for both candidates to speak to a broad audience and to sway voters who remain undecided. With a kind of relish, Mr. Trump abandoned any effort to attract moderate voters to his cause, preferring instead to engage in ugly attacks on Ms. Clinton that would have been unthinkable prior to this campaign.
"Such a nasty woman," he said of Ms. Clinton as she discussed America's Social Security system toward the end of the debate.
Earlier, Mr. Trump called the Clinton Foundation a "criminal enterprise" and asserted Ms. Clinton's use of a private e-mail server was "criminal." He asserted that Ms. Clinton should not have been allowed to run for president because she is "guilty of a very, very serious crime." He also described her campaign as "sleazy" and "crooked."
Ms. Clinton launched her own series of attacks. She noted that Vermont Senator Bernie Sanders, her opponent in the Democratic primary, has called Mr. Trump "the most dangerous person to run for president in the modern history of America." Ms. Clinton went on: "I think he's right."
She also described Mr. Trump's assertions that the election is somehow rigged as "horrifying" and part of a pattern. Mr. Trump, she noted, cried foul every time he believes "things are not going in his direction," whether in the Republican primaries or in his pursuit of an Emmy award for his reality television show.
"Should have gotten it," Mr. Trump interjected, provoking laughter from the audience.
"It's funny but it's also really troubling," Ms. Clinton shot back. "This is not the way our democracy works." She continued: "We've accepted the outcomes when we might not have liked them. … He is talking down our democracy and I, for one, am appalled."
The debate caps another startling stretch in a campaign that has no precedent in modern history. As Mr. Trump grapples with sexual-assault accusations, he has ramped up his assertions that the results of the U.S. election will be manipulated in favour of his opponent.
Such allegations – made without any evidence – have stoked fears that Mr. Trump's supporters will take actions into their own hands and interfere with the voting process on Election Day. Mr. Trump has persisted in his claims even though a host of Republican election officials at the state level have declared their confidence in the electoral process.
Mr. Trump has also claimed the media is biased against him, charges he reiterated in the debate. "The media is so dishonest and so corrupt and the pile on is so amazing," he said. "They poison the minds of voters."
Ms. Clinton, meanwhile, is contending with a massive release of information by WikiLeaks obtained through by an apparent illegal hacking of her senior aide's e-mail. Ms. Clinton's speeches to Wall Street firms, which she sought to withhold, are now public, as are thousands of e-mails sent by John Podesta, the chairman of Ms. Clinton's campaign.
Ms. Clinton reiterated her campaign's assertion that the hacking was orchestrated by Russia, a claim backed up by the White House and American intelligence agencies. "This is such an unprecedented situation," she said. "We've never had a foreign government trying to interfere in our election."
Mr. Trump tried distance himself from Russian President Vladimir Putin. "I don't know Putin. He said nice things about me. If we got along well, that would be good," Mr. Trump said. Then he asserted Mr. Putin had no respect for Ms. Clinton.
"Well, that's because [Putin] would rather have a puppet as president of the United States," Ms. Clinton responded.
"No, you're the puppet," Mr. Trump retorted.
Much like the second debate, Mr. Trump's campaign attempted to use its guest list as a way to rattle his opponent. One of the people his campaign invited to Las Vegas was Leslie Millwee, a woman who came forward on Wednesday to accuse former president Bill Clinton of sexually assaulting her in 1980 when he was governor of Arkansas.
Chris Wallace of Fox News served as moderator for Wednesday's debate, which dispensed with the town-hall format used in the prior face-off in St. Louis, Mo. Known as a tough questioner with a playful streak, Mr. Wallace had surprises for both candidates, interrogating Ms. Clinton about what she said in her speeches to large banks and pinning down Mr. Trump on his position on a landmark abortion case.
While Mr. Trump asserted during the debate that he will win the election, he also alluded to a victory by Ms. Clinton during a discussion of refugees. Mr. Trump said the U.S. is taking in tens of thousands of Syrian refugees who are "definitely in many cases ISIS-aligned." (Both the number and the characterization are false). "Wait until you see what happens in the coming years," he said. "Lots of luck, Hillary."
It's not clear what Mr. Trump can do at this late stage to reinvigorate his campaign. According to the website FiveThirtyEight, which produces an election model based on an aggregate of national and state-level polls, as of Wednesday Mr. Trump had a 14 per cent chance of winning, compared to an 86 per cent chance for Ms. Clinton. Mr. Trump's chances of victory have declined precipitously in the three weeks since the first presidential debate, when he had a 45 per cent chance of becoming the next president according to the model.
Outside the debate hall on the campus of the University of Nevada, Las Vegas, a raucous scene unfolded as supporters of the two presidential candidates waved signs and chanted slogans. "Lock her up! Lock her up!" shouted Mr. Trump's fans. "Hill-ar-y! Hill-ar-y!" responded Ms. Clinton's enthusiasts.
Blake Wassmann, a third-year student from Georgia, proudly wore a T-shirt reading "Hillary for Prison 2016." He is voting in a presidential election for the first time. "I don't believe the polls whatsoever," he said. He added that he believes the election could be rigged, because "government is corrupt on the inside."
Ms. Clinton's supporters sounded confident that victory was near. "We're working hard every day to get her elected," said Lynnette Hull, 18, who is working as an intern with Ms. Clinton's campaign. "We're in the homestretch and I'm feeling pretty confident." She dismissed Mr. Trump's claims of a rigged election. "It's an easy way out to defend himself before he loses," she said.