Republican presidential nominee Donald Trump refused to guarantee he would accept the US presidential election result if he lost, in an unprecedented undermining of democracy that throws into question the peaceful transition of power.

Doubling down on his dark rhetoric that the election was "rigged" in the third and final election debate.

Trump was pressed by the moderator if he would concede the election if voters backed Hillary Clinton on November 8.

Trump said he will keep Americans in "suspense".

"I will not do anything now," Trump said.

"I will look at it at the time. What I have seen is so bad," he added.

The billionaire real estate developer attacked the "corrupt" media's coverage of his unorthodox campaign and claimed there were "millions" of people registered to vote who shouldn't be.

Political analysts were stunned by Trump's recant of his previous pledge to accept the poll outcome and said it would turn off undecided voters who the mogul desperately needs to woo to get his derailing campaign back on track against Clinton.

Concerns are growing that Trump's defiance could inflame a backlash or violence from his diehard supporters if he loses as opinion polls indicate is very likely.

Washington Post conservative political commentator Robert Costa said on MSNBC that Trump was preparing a "grievance populist movement" for post the election.

At the debate in Las Vegas, Clinton fired back at Trump's disrespect for America's proud democracy, arguing it was "horrifying" and "troubling".

"We have been around for 240 years. We have had free and fair elections. We've accepted the outcomes when we may not have liked them. That is what must be expected of anyone standing on a debate stage during the general election," she said.

Trump's unprecedented statement on the election outcome contradicted earlier assurances from his vice presidential running mate Mike Pence, campaign manager Kelleyanne Conway and his daughter Ivanka.

Attacks on democracy

Soon after the debate, the left-leaning The New York Times editorial board wrote that Trump "attacks democracy itself".

Ms Conway tried to deflect from Trump's ambivalence on the poll outcome, arguing that Democrat Al Gore retracted his concession to Republican George W Bush in the 2000 presidential election.

The cliff hanger election 16 years ago rested on a nail biting result in Florida that was ultimately settled by the Supreme Court in favour of Bush.

"Donald Trump will accept the results of the election, he's going to win the election so it will be easy to accept," Ms Conway added.

By raising the spectre of disobedience after the election, Trump's antics overshadowed the broader debate topics of debt and entitlements, immigration, the economy, the Supreme Court, foreign affairs and the candidates' fitness to be president.

Trump also refused to validate the findings of US spy agencies who have accused Russia of hacking into Democratic Party emails and interfering in the election.

Unlike Clinton, he did not condemn Russia and its leader, Vladimir Putin, for attacking American democracy. Clinton lashed Trump for being the "puppet" of Putin. Trump accused Clinton of being a "nasty woman" for her attacks on his lack of tax disclosure.

Political commentator and former strategist for President Obama, David Axelrod, praised Mr Trumps' early debate performance on appealing to conservatives on gun rights and immigration.

But he said the failure to accept an election loss would cost him dearly with undecided voters, labelling it "not American".

"This was a disaster for Donald Trump," Axelrod said.

A CNN instant poll found viewers said Clinton won the debate, by a margin of 52 to 39 per cent.

Underlining the ill feeling between the Republican and Democratic nominees, they did not shake hands before the debate, nor did they appear to embrace after the 90-minute stoush.

Clinton mocked Mr Trump's past claims that various systems were "rigged" when they didn't work in his favour, such as the FBI's benign investigation of her government emails, the Republican primary elections in Iowa and Wisconsin and the independence of the court presiding over his failed Trump University.

She skewered the former reality TV star for complaining the Emmy television awards were fixed when he was passed over for a prize.

Agencies