WASHINGTON (AFP) - White House candidate Donald Trump desperately needs a strong debate performance against rival Hillary Clinton on Sunday (Oct 9), with stakes sky-high following intense scrutiny of his treatment of women and damaging footage of him making lewd remarks.

His unprecedented, outside-the-establishment presidential bid, and the embattled Republican Party with it, was thrown into disarray by his misogynistic comments, with growing calls from top Republicans for him to step aside.

Even before the latest fallout, Mr Trump was already in need of a moment of political magic to reverse his slide in the polls barely four weeks from Election Day on Nov 8.

Now his campaign has been rocked by its worst crisis, with his crude comments echoing in voters' ears, day in and day out.

National media have dug up evidence of some of his bad and bizarre behaviour, including agreeing with an interviewer that his daughter Ivanka was a "piece of ass". In a 2002 interview with Howard Stern, Trump also said he preferred leaving women over a certain age.

"What is it at 35? It's called check-out time," he quips.

At 9pm (9am on Monday Singapore time), the real estate magnate will face the former secretary of state in their second presidential debate which will take place at Washington University in St Louis.

The format poses its own difficulties for Mr Trump: half of the questions will be asked by undecided voters.

He will want to build a personal connection with these everyday Americans and show his capacity for empathy, a quality that often has been absent in his large, raucous campaign rallies.

Despite an angry backlash over Mr Trump's remarks boasting about his ability to grope women as he pleases without impunity, he insisted there is "zero chance I'll quit".

Late on Saturday, the defiant Republican stepped outside of his Trump Tower skyscraper in New York, brandishing his fist to cheers from dozens of supporters.

Asked if he was staying in the race, he responded: "100 per cent".

Mr Trump's own wife Melania said she was offended by her husband's "unacceptable and offensive" comments, caught on a hot mic just months after the two married.

But she urged voters to support him.

"I hope people will accept his apology, as I have, and focus on the important issues facing our nation and the world," she said in a statement.

In the footage, released on Friday by The Washington Post, Mr Trump can be heard using vulgar and predatory language as he describes hitting on a married woman and grabbing women's crotches.

It forced a rare apology from a campaign already peppered by controversies over Mr Trump's treatment of women, roiling his Republican Party.

The Republican National Committee (RNC) appeared to have halted part of its "Victory" programme to elect Mr Trump, asking for a mail production to be "put a hold", the Politico news website reported.

CNN said the RNC was considering ending a joint fund-raising agreement with the Trump campaign.

But Mr Trump called the disclosure a "distraction", defiantly hitting back at the Clintons over husband Bill's past infidelities, and hinting he would say more during Sunday's debate.

Britain's Nigel Farage, who co-founded the UK Independence Party that led this year's shock campaign to leave the European Union, dismissed Mr Trump's remarks as little more than "alpha male boasting" and "the kind of thing men do."

Republican reaction to the video tape came fast and furious, with some calling on him to step aside or allow running mate Mike Pence to take over, while others simply withdrew their endorsement.

Mr Pence, the governor of Indiana, said he was "offended" by Mr Trump's remarks.

But Mr Trump's campaign released a schedule showing the bombastic billionaire would be back on the trail for rallies starting on Monday.

House Speaker Paul Ryan, the top Republican officeholder, said he was "sickened" by Mr Trump's comments, and withdrew an invitation for him to attend a political event in Wisconsin.

By Saturday, about a dozen senators, a dozen members of the House of Representatives and three governors - all Republicans - had withdrawn their support.

Among them was former secretary of state Condoleezza Rice, who said: "Enough! Donald Trump should not be president. He should withdraw." Senator John McCain, the 2008 presidential nominee with whom Mr Trump has sparred repeatedly, said: "Donald Trump's behaviour... make(s) it impossible to continue to offer even conditional support for his candidacy." Governor John Kasich of Ohio said Trump's comments were "disgusting" and Illinois Senator Mark Kirk called for an "emergency replacement".

Actor-director Robert de Niro also weighed in, saying: "I'd like to punch him in the face."

But top Trump surrogate Rudy Giuliani, a former New York mayor, insisted that there was "nothing that would cause his dropping out."

"That is wishful thinking of the Clinton campaign and those people who have opposed him for a long time. He is in the race to win," Mr Giuliani added.

With the Nov 8 elections just a month away and Ms Clinton leading in the polls by nearly five percentage points nationally, the latest uproar has plunged Mr Trump in the deepest crisis of his turbulent campaign.

He had already been seriously hurt by a sloppy performance in his first debate with Ms Clinton on Sept 26, a damaging Twitter war against a former Miss Universe and reports he may have paid no income taxes for 18 years.

Ms Clinton, who is seeking to become the nation's first female commander-in-chief, is almost certain to call out Mr Trump about the footage during the debate.

"This is horrific," she said on Twitter. "We cannot allow this man to become president".