Trump booed in New York, assailed by Obama

Donald Trump endured boos at a charity event in New York and a biting attack by President Barack Obama as the Republican nominee's threat not to concede if he loses next month's elections roiled the race for the White House.

Trump cast the United States into uncharted political waters by suggesting he may not recognize the result of the November 8 presidential election and could launch a legal challenge if Hillary Clinton wins.

"I will totally accept the results of this great and historic presidential election... if I win," the Republican nominee told cheering supporters in Delaware, Ohio.

"Of course I will accept a clear election result, but I will also reserve my right to contest and file a legal challenge in the case of a questionable result," he said dangling his concession.

Trump later was booed at a white-tie fundraiser for Catholic charities in New York where he and Clinton, fresh off their bruising final debate in Las Vegas, took part in what was supposed to be a friendly roast.

But the bitterness of the campaign was soon on display, with Trump calling Clinton "corrupt" and jabbing her for disclosures from her campaign's hacked emails.

"I don't know who they're angry at, Hillary, you or I," Trump said in an attempt to brush off the ensuing flurry of boos from New York's elite.

Obama's rebuke

The 70-year-old billionaire is trailing badly in the polls, and his performance debate opened him up to a stinging attack from Obama at a rally in Miami.

"When you try to sow the seeds of doubt in people's minds about the legitimacy of our election, that undermines our democracy," Obama said.

"When you suggest rigging or fraud without a shred of evidence, when last night at the debate, Trump becomes the first major party nominee in American history to suggest that he will not concede despite losing... that is not a joking matter."

The unusually harsh comments suggest the White House believes this deeply rancorous election is not just about defeating Trump or winning back control of Congress, but snuffing out his populist credo.

The reality TV star has defied political convention and brought far-right policies and conspiracies to the Republican mainstream.

The final 2016 presidential debate on Wednesday was dominated by Trump's refusal to say he would recognize a victory by Democrat Clinton, 68, who he accuses of conspiring to rig the vote against him.

"There is no way to rig an election in a country this big," Obama fired back. "You are much likelier to get struck by lightning than have somebody next to you commit voter fraud."

His wife Michelle encouraged voters not to heed the Trump campaign's rigging allegations.

"They are trying to convince you that your vote doesn't matter," she told a Phoenix rally. "That the outcome has already been determined, and you shouldn't even bother to make your voice heard."

She added: "They are trying to take away your hope."

Vote rigging

Although Trump looks set to lose the election, his campaign sent into a tailspin by a stream of allegations of sexual misconduct, he is likely to garner as many as 50 million votes.

Despite isolated allegations of voter fraud, controversy over the tight 2000 vote and rampant gerrymandering, US elections have been regarded as free and fair.

Asked point-blank by a debate moderator on Wednesday whether he would accept the election result if he lost, the reality television star shattered that consensus.

"I'll look at it at the time. What I've seen is so bad," he said, repeating unfounded allegations of vote rigging.

Asked again by the moderator, Trump said "I'll tell you at the time. I'll keep you in suspense, OK?"

His rival on the debate stage, and a phalanx of his fellow Republicans rushed to tell Trump it was not "OK."

Clinton declared herself "appalled" by what she said was an attack on 240 years of US democracy.

Trump's vice presidential running mate Mike Pence insisted "we'll accept the will of the American people."

Reince Priebus, the chairman of the Republican National Committee, offered assurances. "Barring massive voter fraud, of course he is going to accept the results of the elections," he said.

Polls show Clinton leading by more than six points and making gains even in states like Arizona, Texas and Georgia that have long been in the Republican column.