“I will totally accept the results of this great and historic presidential election -- if I win,” he told cheering supporters in Delaware, Ohio on Thursday, appearing to revel in the controversy he generated the previous day at their third and final presidential debate when he refused to pledge to honor the outcome.

“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 told his Ohio rally.

The pair also are coming off an evening of stinging humour at a white-tie charity event in New York where they traded colourful barbs at what is meant to be a friendly roast -- and where Trump was booed.

The bitterness of the campaign was quickly on display, with Trump calling the 68-year-old Clinton “corrupt” and jabbing her for disclosures from her campaign’s hacked emails.

“Here she is in public, pretending not to hate Catholics,” he said, as Cardinal Timothy Dolan of New York looked on.

“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.

Despite the prickly barbs -- including Clinton proclaiming that Trump sent “a hearse” to bring her to the dinner -- the two candidates shook hands at the end of the evening.

Trump is trailing badly in the polls, and his debate threat opened him up to a stinging attack from President Barack Obama at a Miami rally.

“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 Thursday.

“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.”

With 18 days to go, Trump is seeking to dig out wins in key states. But Clinton is well aware that no Republican has ever won the White House without winning Ohio, and she returns to the Buckeye State in a bid to halt Trump in his tracks.

“We feel very good about where things stand,” David Pepper, chairman of the Ohio Democratic Party, told CNN prior to Clinton’s arrival, referring to state polls that had favoured Trump but have since tightened into a virtual tie.

“Our ground game is robust; theirs is non-existent,” he said.

In several other battleground states, Clinton holds leads ranging from razor-thin, such as in North Carolina, to moderate in Florida and Pennsylvania and commanding in Virginia.

She is even narrowly ahead in Arizona, the traditionally Republican-leaning state where First Lady Michelle Obama -- who galvanised voters with a searing attack on Trump last week -- campaigned for Clinton on Thursday.

If Trump loses Florida, Pennsylvania and North Carolina, Clinton is all but assured of victory, experts have said.

New York Times election forecast team The Upshot put Clinton’s chances of winning at 93 per cent.

Trump, a onetime reality TV star, has defied political convention and brought far-right policies and conspiracies to the Republican mainstream, but his campaign was sent into a tailspin in recent weeks by a stream of allegations of sexual misconduct.

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 the debate moderator Wednesday whether he would accept the election result if he lost, Trump shattered that consensus.

“I’ll keep you in suspense, OK?” he said.

Trump’s running mate Mike Pence insisted “we’ll accept the will of the American people.”