First lady Michelle Obama criticized Trump in scathing terms in a campaign speech for Clinton in New Hampshire on Thursday.

Donald Trump assailed as "absolutely false" the allegations by several women that he groped them, and blamed Democratic rival Hillary Clinton, the media and lobbyists of being engaged in a vicious effort to stop him from winning the White House.

With his numbers dropping in opinion polls only weeks before the Nov. 8 election, the Republican presidential nominee told supporters at a rally in Florida that his campaign was engaged in "a struggle for the survival of our nation."

Trump said accusations that he groped women in a series of incidents going back to the 1980s were part of a coordinated attempt to keep him from the Oval Office.

"These vicious claims about me of inappropriate conduct with women are totally and absolutely false," he said, adding that "the Clintons know it." He said he would make public at some point evidence to dispute the charges.

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Trump spoke after The New York Times reported that two women said they had endured sexual aggression from him, and several other women made similar allegations in other media outlets.

The New York businessman's campaign was already struggling to contain a crisis after a video surfaced last week showing him bragging in 2005 about groping women and making unwanted sexual advances.

First lady Michelle Obama criticised Trump in scathing terms in a campaign speech for Clinton in New Hampshire on Thursday.

Her voice close to cracking with emotion, Obama described the Republican as "a powerful individual speaking freely and openly about sexually predatory behavior."

"It's one of countless examples of how he has treated women his whole life and I have to tell you that I listen to all this and I feel it so personally," she said.

One woman, Jessica Leeds, appeared on camera on The New York Times' website to recount how Trump grabbed her breasts and tried to put his hand up her skirt on a flight from the Midwest to New York in or around 1980.

The second woman, Rachel Crooks, described how Trump "kissed me directly on the mouth" in an unwanted advance in 2005 at Trump Tower in Manhattan, where she was a receptionist at a real estate firm.

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Reuters could not independently verify the incidents. Leeds and Crooks did not respond to requests for comment from Reuters.

The New York Times said on Thursday it stood by its story and rejected charges the article was libelous after a lawyer for Trump threatened legal action and demanded a retraction.

Slump in polls

Trump has slumped in opinion polls in recent days as uproar over the video threatened to engulf the former reality TV star's White House campaign in a way that earlier controversies surrounding him have not.

A Reuters/Ipsos survey showed one in five Republicans thought Trump's comments about groping women disqualified him from the presidency. The poll also showed him 8 points behind Clinton among likely voters.

Trump trails Clinton by up to 11 points in other polls.

He has never previously run for political office. Provocative statements have been a feature of his presidential bid since the day in June 2015 when he described Mexican immigrants as rapists and criminals in a speech announcing he was running for president.

Establishment Republicans have struggled to get behind Trump, alarmed by both his style and some of his policy proposals.

Within hours of The New York Times report, several other media outlets published similar reports. People magazine published a detailed first-person account from one of its reporters, Natasha Stoynoff.

Stoynoff said Trump pinned her against a wall at his Florida estate in 2005 and kissed her as she struggled to get away.

"I turned around, and within seconds, he was pushing me against the wall, and forcing his tongue down my throat," Stoynoff said.

Trump, 70, denied the People story in a Twitter message and in his speech in Florida on Thursday, and mocked the writer.

"I ask her a simple question. Why wasn't it part of the story that appeared 12 years ago? Why didn't they make it part of the story ... if she had added that, it would have been the headline."

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"Look at her and look at her words," he said. "You tell me what you think. I don't think so."

The Palm Beach Post reported an allegation by Mindy McGillivray, 36, a woman in South Florida, that Trump had grabbed her bottom 13 years ago while she was working at his Mar-a-Lago estate as a photographer's assistant.

"There is no truth to this whatsoever," Trump's spokeswoman, Hope Hicks, told the Post. McGillivray could not be reached for comment.

At the rally on Thursday, Trump said he had been prepared for attacks, but "I never knew it would be this vile, that it would be this bad, that it would be this vicious."

He said the "corrupt political establishment," which he said included special interests, was trying to stop him so it could carry out a program of "radical globalisation and the disenfranchisement of working people."

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"Our great civilisation, here in America and across the civilized world has come upon a moment of reckoning," he said.

Republican hopes

In the 2005 video, Trump bragged about groping women, kissing them without permission, and trying to seduce a married woman. He said during a presidential debate on Sunday that he had not actually done the things he had boasted about, and apologized for his remarks, which he called private "locker room talk."

Fallout from the video has thrown a cloud over Republican hopes of retaining control of the US Congress and deeply split the party as elected officials have abandoned support for the candidate.

A spokeswoman for Clinton, 68, said Wednesday's report in The New York Times was "disturbing."

The Washington Post endorsed Clinton, a former secretary of state, on Thursday. "In the gloom and ugliness of this political season, one encouraging truth is often overlooked: There is a well-qualified, well-prepared candidate on the ballot ... we endorse her without hesitation," it said.