Presidential candidates Hillary Clinton and Donald Trump wasted no time in their second debate Sunday night before talking about the 2005 recording of his lewd talk about groping women, which has thrown his Republican Party into turmoil.

The two candidates avoided shaking hands as they walked onto the debate stage in St Louis, Missouri, and immediately launched into harsh personal attacks.

In the recording, released Friday, the New York real estate tycoon speaks lewdly of grabbing women "by the pussy."

One of the debate moderators, CNN news anchor Anderson Cooper, told Trump: "You bragged that you have sexually assaulted women."

"I didn't say that at all," Trump said, before changing the topic to the Islamic State terrorist group, known as ISIS.

"Yes, I'm very embarassed by it. I hate it, but it's locker room talk, and it's one of those things. I will knock the hell out of ISIS. We're gonna defeat ISIS. ... We should get on to much more important things."

Pressed by Cooper - "Have you ever done those things?" - Trump replied: "No, I have not."

Since Friday, dozens of prominent Republicans, including some of the conservative party's top members of Congress, have repudiated Trump over the video. Many have withdrawn their support for him as the Republican nominee.

"What we all saw and heard on Friday was Donald talking about women: what he thinks about women, what he does to women," Clinton said.

"He has said that the video doesn't represent who he is. But I think it's clear to anyone who heard it that it represents exactly who he is."

She sought to exploit the widening rift between Trump and the party whose nomination he won as an outsider candidate.

"With prior Republican nominees for president, I disagreed with them on politics, policies, principles, but I never questioned their fitness to serve," Clinton said.

Even as Trump sought to put the controversy behind him, he was quick to exploit the sexual dalliances of former president Bill Clinton in an attempt to unnerve and smear his own opponent.

"If you look at Bill Clinton, mine were words and his was action," Trump said, accusing Hillary of attacking the same women.

Fewer than two hours before the debate, Trump held a press conference with four women who described their personal grievances with the Clintons, including three who made previous allegations of sexual assault or harassment against Bill Clinton. Trump later pointed to the women during the debate.

One of the women, Juanita Broaddrick, accuses ex-president Bill Clinton of a 1978 attack and the allegations have followed Clinton for decades though no charges were ever filed.

"Mr Trump may have said some bad words, but Bill Clinton raped me, and Hillary Clinton threatened me," Broaddrick alleged. "I don't think there's any comparison."

Trump had insinuated after a previous debate earlier this month that he had refrained from bring up Bill Clinton's infidelity, and while the move may have further alienated Trump's opponents it could also serve to energize some Republicans by reminding them of the scandals of the Clinton presidency.

A survey by ABC News and polling company SSRS, released Sunday before the debate, found since the release of the lewd video on Friday that 43 per cent of respondents said Trump should quit the race, while 57 per cent said Trump should stay in the race.

Vice presidential candidate Mike Pence, who had distanced himself from Trump over the offensive video, returned to praising Trump for "a big debate win" even as Trump suggested during the debate that the pair had not spoken about key policy issues.

The debate also hit on other controversies with Trump admitting using legal write-offs to avoid paying taxes after a 1995 business loss and saying he was not "unproud" of using Twitter after attacking a former Miss Universe.

Trump meanwhile sought to hit Clinton over her use of a private email server, her response to the terrorist attack on the US consulate in Benghazi and other issues.

"She's got bad judgment, and honestly, so bad that she should never be president of the United States," he said.

Clinton repeated charges that it was "good that someone with the temperament of Donald Trump is not in charge of the law in our country."

"Because you'd be in jail," he shot back, and went on to attack her use of a private email server while secretary of state from 2009-13.

Clinton repeated that it was a mistake for her to use the email server.

"There's a lot at stake," Clinton said. "This is not an ordinary time, and this is not an ordinary election."

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