Asked if he would concede a loss to Hillary Clinton, Republican candidate said "I will look at it at the time".
Donald Trump, the Republican Party nominee for US president, has refused to say that he would accept the election result if he loses, as he clashed with rival Hillary Clinton in their third and final debate.
Declining to be drawn on what he would do, he said: "I will look at it at the time."
Trump has leaned on an increasingly brazen strategy in the campaign’s closing weeks, including peddling charges that the election will be rigged, despite no evidence of widespread voter fraud in previous US presidential contests.
"The biggest issue [from the debate] is his unwillingness to accept the outcome of the election," the Reverend Jesse Jackson, a prominent civil rights leader and member of the Democratic Party, told Al Jazeera. "That could sabotage the entire American process."
Al Jazeera’s Alan Fisher, reporting from Las Vegas, said that it was the first time in three debates that "we saw real policy differences" between the two candidates.
"They argued about them in substantive terms," Fisher said.
One of the hotly debated topics on stage was immigration, which has been a key issue in Trump’s campaign. He repeated a pledge that if he becomes president, a wall will be built on the Mexico border to stop people entering the country illegally.
After discussing the wars in Syria and Iraq, the discussion turned to refugees in need of protection. While Trump repeated his claim that the US does not know who it is letting into the country, Clinton said: "I am not going to let anyone into this country who is not vetted ... but I am not going to slam the door on women and children."
But D’Angelo Gore, a fact-checker with the website Politifact, told Al Jazeera that both candidates mischaracterised each other’s policy proposals during the debate.
"Trump said that Clinton’s immigration policy was to simply grant amnesty to all the immigrants living in the United States, which is inaccurate," Gore said. "The reality is that Clinton’s immigration reform is much more comprehensive, including increased border control."
According to Gore, Clinton’s accusation that Trump wanted to abolish the North Atlantic Treaty Organization was also inaccurate, as Trump has thus far only expressed frustration that NATO has not focused enough on "fighting terrorism" as well as the notion that the US carries most of the financial burden.
Wednesday’s face-off at the University of Nevada came as early voting was already under way in more than 30 states - at least 2.1 million voters have cast ballots already.
For Trump, the debate was perhaps his last opportunity to turn around a presidential race that appears to be slipping away.
In an average of national polls, Clinton has a lead at 48.6 percent over Trump’s 42.1 percent.
His predatory comments about women and a flood of sexual assault accusations have increased his unpopularity with women and limited his pathways to victory.
Discussing the sexual assault claims in the debate, Trump said he did not apologise to his wife, because he "didn’t do anything".
Clinton took the stage with challenges of her own.
While the electoral map currently leans in her favour, she is facing a new round of questions about her trustworthiness, concerns that have trailed her throughout the campaign.
The hacking of her top campaign adviser’s emails revealed a candidate who is averse to apologising, can strike a different tone in private than in public, and makes some decisions only after political deliberations.
When the moderator brought up quotes from a Clinton email released by WikiLeaks in which she seemed to express a stance on trade that differs from what she has said publicly, she quickly deflected the question.
She proceeded to say, "What’s really important about WikiLeaks is that the Russian government has engaged in espionage against Americans ... this has come from the highest levels ... from Putin himself ... to influence this election".