WASHINGTON -- Threatening to upend a fundamental pillar of American democracy, Donald Trump refused to say in the final debate that he will accept the results of next month's election if he loses to Hillary Clinton. The Democratic nominee declared Trump's resistance "horrifying."

Trump's comments at Wednesday's debate sent shockwaves through the presidential campaign. They sharply contrasted with the unequivocal assurances given by running mate Mike Pence and other top figures in the campaign that Trump, despite raising unsubstantiated concerns about voter fraud, would accept the results.

Asked whether he would accept the election outcome, Trump replied: "I will tell you at the time. I'll keep you in suspense,"

Trump's assertions raise the prospect that millions of his supporters may not accept the results on Nov. 8 if he loses, thrusting the United States into uncharted territory. Free and fair elections, with the vanquished peacefully stepping aside for the victor, have been the underpinning of America's democratic tradition since the country's founding.

"Donald Trump is trying to pull the central pillar down," Clinton running mate Tim Kaine said Thursday on MSNBC.

Even some of Trump's supporters felt it was a step over the line. Conservative commentator Laura Ingraham said her preferred candidate "should have said he would accept" the election's results." The Republican National Committee was put in the remarkable position of disputing its own nominee, insisting that the GOP would "respect the will of the people."

There is no evidence of widespread voter fraud, and election officials across the country from both parties have denied and denounced Trump's charges.

Trump had entered the debate trailing in nearly all battleground states and needing a strong performance to change the trajectory of the race with less than three weeks before Election Day and with early voting underway in more than 30 states. He emerged once again on the defensive over a controversy that distracted from his case for the presidency and his focus on Clinton's weaknesses. Though Trump's answer probably would not alienate core supporters who have driven his campaign, it was unlikely to help draw in additional voters whose support Trump sorely needs if he's to win.

Republicans hoping to avoid a thrashing in congressional races were struggling anew with how to distance their candidates from their party's presidential nominee. Democrats wasted no time before vowing to force every GOP candidate to answer for Trump's comments.

The Republican's stunning comments overshadowed the rest of the 90-minute debate, which began calm and policy-focused, but devolved into a bitter and deeply personal confrontation, hewing to the pattern of the previous two face-offs. Trump called Clinton a "nasty woman," while the Democrat panned him as "unfit" to be commander in chief.

Clinton forcefully accused Trump of favouring Russia's leader over American military and intelligence experts after the Republican nominee pointedly refused to accept the U.S. government's assertion that Moscow has sought to meddle in the U.S. election.

She charged that Russian President Vladimir Putin was backing Trump because "he'd rather have a puppet as president of the United States."

Trump denied any relationship with Putin and said he would condemn any foreign interference in the election. But he notably declined to back the intelligence community's assessment that Russia was involved in the hacking of Democratic organizations. The Clinton campaign has said the FBI also is investigating Russia's involvement in the hacking of a top adviser's emails.

Though the candidates won't appear on the debate stage again, they'll have at least one more run-in on Thursday when they sit just one seat apart at the Alfred E. Smith Memorial Foundation Dinner, a white-tie gala in New York. Having mocked and insulted each other throughout the campaign, Trump and Clinton must try to lighten the mood at the roast, in which candidates traditionally ditch the harsh attacks and try to be funny.

In the debate, the candidates clashed repeatedly over their drastically different visions for the nation's future. Trump backed Supreme Court justices who would overturn the landmark Roe vs. Wade ruling, while Clinton vowed to appoint justices who would uphold the decision legalizing abortion, saying, "We have come too far to have that turned back now."

The businessman entered the final debate facing a string of sexual assault accusations from women who came forward after he denied in the previous contest that he had kissed or groped women without their consent. That Trump denial followed the release of a video of in which he was heard bragging about exactly that.

Trump denied the accusations anew Wednesday night, saying the women coming forward "either want fame or her campaign did it." He falsely said the women's allegations had been debunked.

Clinton said Trump "thinks belittling women makes him bigger. He goes after their dignity, their self-worth." She avoided answering a question about her husband's infidelities.

Despite Clinton's favourable electoral map, she has struggled throughout the campaign to overcome persistent questions about her honesty and trustworthiness. In the election's closing weeks, she's begun appealing to Americans to overcome the deep divisions that have been exacerbated by the heated campaign, saying onstage Wednesday that she intended to be a president for those who vote for her and those who do not.

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AP writers Darlene Superville and Josh Lederman contributed to this report.