Donald Trump and Democrat Hillary Clinton head into a crucial presidential debate Sunday with the Republican's White House campaign in chaos over his lewd boasts about groping women.

With the US election less than a month away, Republican lawmakers and governors abandoned Trump in droves, despite a rare televised apology by the candidate, whose sexually aggressive remarks were caught on a hot mic in 2005.

Trump himself was defiant, posting online a video of women who have alleged they were sexually assaulted by former president Bill Clinton.

"So many self-righteous hypocrites. Watch their poll numbers -- and elections -- go down!" Trump tweeted.

But even his surrogates went to ground, leaving former New York mayor Rudolf Giuliani as the billionaire's lone defender on Sunday television talk shows.

Trump campaign manager Kellyanne Conway and Republican National Committee chairman Reince Preibus both backed out of scheduled television appearances ahead of the debate in St Louis, Missouri.

During a campaign event for Democrats in his home state of Illinois, President Barack Obama weighed into the political crisis roiling both Trump's campaign and the Republican Party as a whole, saying the man vying to replace him was "insecure."

"Demeaning, degrading women, but also minorities, immigrants, people of other faiths, mocking the disabled... That tells you a couple things," Obama said.

"It tells you that he is insecure enough that he pumps himself up by putting other people down. Not a character trait that I would advise for somebody in the Oval Office."

Mike Pence, Trump's running mate, also suspended his campaign appearances after declaring on Saturday that he "cannot defend" Trump's remarks.

The scandal -- just the latest involving his treatment of women -- could not come at a worse time for Trump, who has taken a beating in the polls since his sloppy performance in the first presidential debate September 29.

An average of national polls compiled by RealClearPolitics.com has Clinton in the lead by 4.5 percentage points.

National media, meanwhile, have dug up the candidate's past behavior, including agreeing with an interviewer that his daughter Ivanka was a "piece of ass."


Debate with a twist

The two candidates face off starting at 8:00 pm (0100 GMT Monday) at Washington University in St Louis, with the added twist that this time they will take questions from the audience in a town hall-style forum.

"He's as prepared as he's ever been and he's all ready for the debate tonight," Giuliani said on NBC's "Meet the Press."

"He obviously, you know, feels very bad about what he's said. He apologized for it, will probably do it again. What he'd like to do is move onto the issues that are facing the American people."

How much the "elephant in the room," as Republican House Speaker Paul Ryan referred to the tape on Saturday, dominates the debate is an open question.

Giuliani warned that Trump could well go after Clinton's past marital troubles with her husband Bill.

"I do believe there's a possibility he'll talk about Hillary Clinton's situation if it gets to that. I don't think he prefers to do that. But I think he will," Giuliani said.

Jennifer Palmieri, Clinton's communications director, told reporters on her flight to St Louis the town-hall format in the second presidential debate plays to the former first lady's strengths.

But she added: "We understand this is uncharted territory to face an opponent that is in the grips of a downward spiral in terms of his own party belatedly walking away from him.

"She has a lot of experience, she is very tough, she will be prepared to handle whatever comes (her) way but we find it hard to predict what that might be," Palmieri said.

Clinton may also have to worry about a WikiLeaks disclosure of excerpts from private speeches she gave to major banks in 2013 and 2014.

Although overshadowed by the Trump tape, they show she expressed views in favor of open trade and Wall Street self-regulation to those audiences that are at odds with her positions as a candidate.

John Podesta, a Clinton adviser from whose email account the excerpts were hacked, insisted they were taken out of context.

"They are not diametrically opposed," he said on "Fox News Sunday."

"Again, you can pull a few words out of context, but what she said on this campaign trail is she'll be tough on Wall Street. That's exactly what she'll do."


Republican breach

Even a winning performance by Trump, however, seems unlikely to mend the deep breach he has opened in the Republican party, alarmed about the scandal's fallout in other down-ballot races.

Notable defectors included Senator John McCain, the 2008 Republican presidential nominee, who said Trump's "demeaning comments about women and his boasts about sexual assaults, make it impossible to continue to offer even conditional support for his candidacy."

The New York Times reported that by late Saturday, 36 Republican members of Congress and governors had disavowed Trump's candidacy.

Some leading Republicans called on Trump to quit the race, but he vowed he would never step aside, and legal experts said removing him from the top of the ticket would be extremely difficult.

Trump predicted Saturday that the controversy would blow over.

"I think a lot of people underestimate how loyal my supporters are," he told the Times.