Asked by debate moderator Chris Wallace whether he would accept the election results, Trump said some version of "I will look at it at the time" repeatedly.
Fox News Sunday host Chris Wallace, who served as one of three Fox News moderators during a January 28 Republican primary debate.
Trump's response? "I'll keep you in suspense".
It was after the emergence of the video that Trump's presidential campaign has been jolted with Clinton taking a substantial lead over him in national opinion polls. We are not, and our 240 years as a republic are a testament to that fact. We've had free and fair elections.
While occasionally wordy, Wallace's questions seemed created to get the candidates talking and he mostly let them go at it.
It was a stunning moment, not just because, as Vox's German Lopez pointed out, direct personal insults like that are exceedingly rare on a presidential debate stage.
Later she told reporters: "What he said tonight is part of his whole effort to blame somebody else for where he is in his campaign".
Trump is also facing accusations of sexual assault and inappropriate touching, with as many as nine women coming out with allegations against him.
So while the hashtag #nastywomenvote has been trending due mostly to women getting in a few jokes about Trump's comment, it is following traditional paths of women's rights groups. U.S. Senator Lindsey Graham, a former Republican presidential candidate who has never warmed to Trump, said: "If he loses, it will not be because the system is "rigged" but because he failed as a candidate".
"Barring massive voter fraud, of course he is going to accept the results of the elections", he said. "Confirms one's judgment he shouldn't be president".
Trump's campaign manager Kellyanne Conway, defended the candidate Thursday against charges of undermining U.S. democracy.
"We'll have to see what happens", she told ABC News, adding that Trump "was willing to accept the election absent widespread fraud".
Trump left Las Vegas immediately after the debate, heading to swing state OH for a day of campaigning.
Clinton's running mate, Virginia Senator Tim Kaine, said Thursday that Trump's comments were "a huge shocker", on CBS This Morning.
Trump is widely unpopular among Mexicans due to his disparaging remarks about immigrants and his repeated vows to build a border wall and make their country pay for it.
We invite Mr. Trump, Secretary Clinton, Gov. Johnson, Jill Stein and all candidates for U.S. House and Senate to forcefully reject these irresponsible conspiracy theories or attempts to suppress the vote, endorse the nine nonpartisan back-up steps states can take now to ensure every vote counts, and accept the outcome once a victor is clear.
As he entered the campaign's final stretch on Thursday, Trump tried to turn the tables on Hillary Clinton by accusing her of "cheating" and questioning whether Clinton should "resign from the race". It also echoes his declaration at Clinton in the last debate that, if he won, he would "instruct my attorney general to get a special prosecutor to look into your situation", even though FBI Director James Comey insisted that no reasonable prosecutor would have brought a case against her based on the State Department email scandal.
Trump has spent the past week denying and defending himself from charges that he groped women. She said Trump had in the past also complained that his TV show was unjustly denied a USA television Emmy award.
"I should have gotten it", Trump retorted.
"You've got to listen to everything he said", she argued.
"Donald thinks belittling women makes him bigger", Hillary Clinton said at the debate on October 19, 2016. In 2004, the candidates did not attend after the Catholic Archdiocese of NY agonized over whether Democratic nominee John Kerry, a Catholic who supports abortion rights, should be invited.
Clinton declared herself "appalled" by what she said was an attack on 240 years of USA democracy.
As she continued talking Trump interjected, "Such a nasty woman".
A third of Trump's own backers said he should accept the election results, while 36 per cent say he shouldn't. Typically, party nominees shake hands with each other at the beginning of the event as a display of respect.
The two candidates are scheduled to meet again later on Thursday night at a charity dinner in NY.