US Republican presidential candidate Donald Trump has questioned the legitimacy of the election, pressing unsubstantiated claims the contest is rigged against him.

Key points: Donald Trump says the US election is being "rigged" by a "corrupt media"

Donald Trump says the US election is being "rigged" by a "corrupt media" Mr Trump has denied all allegations of groping women

Mr Trump has denied all allegations of groping women Says he and Hillary Clinton should take a drug test before the next debate

Mr Trump challenged accusations from several women that he made unwanted sexual advances toward them.

Trump spokesman Jason Miller denied a new allegation that surfaced on Saturday, saying there was "no way" Mr Trump had kissed a woman without permission 20 years ago at his Florida resort.

Mr Trump told an outdoor rally in New Hampshire on Saturday that the media was sharing the accusations against him to boost Democrat Hillary Clinton and damage his own White House hopes.

Mr Trump has presented no evidence for claims he has made for months that the election could be fixed to prevent him from reaching the Oval Office. Leaders on both sides of the political spectrum worry that this rhetoric, which supporters sometimes repeat in interviews, undermines the democratic process.

"It looks to me like a rigged election," Mr Trump said in New Hampshire.

"The election is being rigged by corrupt media pushing completely false allegations and outright lies in an effort to elect her president."

He started the day tweeting several times that the election was being swayed by a media conspiracy.

"This election is being rigged by the media pushing false and unsubstantiated charges, and outright lies, in order to elect Crooked Hillary!" he said in one Twitter post. He reiterated similar claims at a later rally in Bangor, Maine.

Sorry, this video has expired Trump calls for drug testing before third debate

Clinton leading Trump in polls

Mrs Clinton, a former US secretary of state, led Mr Trump by seven percentage points in the most recent Reuters/Ipsos national opinion poll of voters.

A new analysis found she was heavily favoured to reach the 270 Electoral College votes needed to secure the presidency.

Mr Trump's campaign said on Saturday it had raised a total of $US100 million in September, mostly from small donors but also including a $US2 million monthly contribution from Mr Trump.

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At the same time, the campaign squabbled with the Republican party in Ohio, a key swing state in the November 8 election, whose Republican leaders have not been shy about concerns with Mr Trump.

On Saturday, Mr Trump's Ohio state director released a letter saying that Ohio Republican Party Chairman Matt Borges "no longer has any affiliation" with the campaign.

Mr Borges responded that the state party had been actively helping Mr Trump there.

Mr Trump's speech in New Hampshire was intended to focus on a plan to end deaths caused by opioid overdoses, something he often mentions on the campaign trail. But he deviated from policy proposals to air his allegations of election-rigging and his response to new groping accusations, 24 days before the election.

Sorry, this video has expired Donald Trump says he is a 'victim' of a 'political smear campaign'

'I think we should take a drug test'

Mr Trump has been fending off the accusations since the release of a 2005 video in which he was recorded bragging about making unwanted sexual advances toward women. Mr Trump has said the boasts were merely words and he has denied each of the allegations.

The latest woman to come forward, Cathy Heller, 63, of New York, told the Guardian newspaper that in or around 1997, Mr Trump kissed her on the lips upon first meeting her during a Mother's Day brunch at his Florida estate. She told the newspaper she leaned away, then turned her head, and he kissed the side of her mouth.

Reuters could not confirm the allegations by Ms Heller, who has contributed to Mrs Clinton's campaign. The Guardian said it spoke to a relative who saw part of the interaction.

"There is no way that something like this would have happened in a public place on Mother's Day at Mr Trump's resort," Mr Miller, the Trump spokesman, said in a statement.

"It would have been the talk of Palm Beach for the past two decades."

On Saturday, Mr Trump deemed "crazy" another woman, Jessica Leeds, now 74, who said he groped her on an airplane in or around 1980.

He also denied allegations of unwanted contact by Summer Zervos, a former contestant on Mr Trump's show The Apprentice, who said he kissed her during a meeting about a possible job.

Mr Trump said Ms Zervos' first cousin called her a "huge fan of Donald Trump", referring to a letter the campaign released to media.

Mr Trump also told the New Hampshire crowd that he and Mrs Clinton were "like athletes" gearing up for the final presidential debate on Wednesday in Las Vegas, and he implied Mrs Clinton might have been on drugs during their last match-up.

"I think we should take a drug test prior to the debate," Mr Trump said.

"In the beginning of her last debate she was all pumped up at the beginning and at the end she was like uh, take me down."

Reuters