Obama calls for 'big' win to repudiate 'dangerous' Trump

"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.

The bombastic 70-year-old is trailing badly in the polls. But Democrats are showing no quarter.

"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 fall trap to 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."

"They are trying to take away your hope."

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.

How his supporters react is now foremost in the minds of officials in the White House and beyond.

It is unclear what impact Trump's stance will have on the election itself.

"Calling an election rigged doesn't just undermine foundational democratic norms and principles, it also reduces voter engagement," said Adam Seth Levine, a professor of government at Cornell University.

'Beyond the pale'

The campaign between Trump and Clinton has proven to be the caustic coda to decades of partisan rancor in Washington.

Political tribalism has already caused gridlock in government and eroded democratic institutions from the Supreme Court to Congress.

But 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.

Roast

Trump and Clinton will later on Thursday attend the same annual charity dinner in New York, an event where the candidates traditionally engage in a "friendly roast."

But the animosity between them seems almost certain to get in the way.

They would not even shake hands at Wednesday night's debate, and at one point Trump interrupted Clinton to call her "a nasty woman".

Clinton, who is vying to become the first woman president of the United States, told reporters she was "both relieved and very grateful" that the debates were now behind her.

Polls show her 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.