COLUMBUS (AFP/REUTERS) - An invigorated Hillary Clinton commanded a record crowd of more than 10,000 supporters on Monday (Oct 10) at a campaign rally, leaving her rival Donald Trump floundering as top elected Republican Paul Ryan all but conceded the White House.

The former secretary of state looking to make history as America's first woman commander-in-chief held an evening rally at Ohio State University ahead of the state's deadline to register to vote on Tuesday.

The turnout marked a record for her campaign, with Mr Trump teetering on the precipice after damaging revelations of his lewd comments about women.

The Clinton camp estimated the size of the crowd at 18,500, including 5,000 outside the perimetre. An AFP reporter said the number was more than 10,000.

The Democrat tried repeatedly to get under her Republican opponent's skin, mocking his television career.

"On the day that I was in the Situation Room watching the raid that brought Osama bin Laden to justice, he was hosting Celebrity Apprentice," she said, a day after the two candidates held their second presidential debate watched by an estimated 66.5 million people.

"So if you want to talk about what we've been doing the last 30 years: Bring. It. On," she added.

The Democrat, who has struggled to energise young voters, focused her speech on youth turnout, repeatedly stressing the stakes of the Nov 8 election, just four weeks away.

"This is turning the clock back not just a few years but centuries. The only way to rebuke this is to vote," she repeated.

Her rival's candidacy suffered a crippling blow after the 2005 tape was released on Friday in which he claimed he could grab women by the crotch with impunity because, as a celebrity, "you can do anything".

The fallout saw a wave of Republican lawmakers abandon him, including some who have urged him to step aside.

Mr Ryan, the top Republican in the US Congress, took the extraordinary step on Monday of distancing himself from Mr Trump, stirring a backlash from some lawmakers and deepening a crisis over his party's struggling presidential nominee.

In a conference call with congressional Republicans, Mr Ryan all but conceded that Mrs Clinton was likely to win the White House on Nov 8 and said he would put his full energy into preserving Republican majorities in Congress so as not to give her a "blank cheque".

Mr Ryan, the speaker of the House of Representatives, said he would not defend Mr Trump or campaign for him after the uproar over the New York businessman's sexually aggressive comments.

A new NBC News/Wall Street Journal poll, conducted after the video was released but before Sunday's debate, showed Mrs Clinton with an 11-point lead in a four-way election - 46 per cent to 35 per cent - and a 14-point lead in a head-to-head match-up.

"I may be limping across that finish line, but we're going to get across," conceded Mr Trump at a rally in Wilkes-Barre in the crunch state of Pennsylvania, for once conceding the scale of the fight before him.

He promised to make six campaign stops a day in the final week before the election, insisting there was still a path to victory and urging his core supporters to come out and vote on Nov 8.

"We have to make sure this election is stolen from us and not taken away from us," he said.

It was the customary rock-star reception for the Republican nominee at the Mohegan Sun Arena in Wilkes-Barre, which has a maximum capacity of 10,000 and was close to - but not entirely - full.

"Without the media, Hillary Clinton couldn't be elected dog catcher," said Mr Trump, calling US television network CNN "a disgrace". CNN host Anderson Cooper was one of the two moderators at Sunday's debate.

At one point, Mr Trump picked up a toddler dressed as a mini-Trump in a grey suit, red tie and white shirt, with blond hair.

"Trump," replied the child into the microphone to laughs when the Republican candidate asked whether the boy wanted to go back to his parents or stay with the nominee, before he handed him back over.

But as he promised to bring back jobs, end illegal immigration, renegotiate trade deals and reduce taxes, one man shouted "how are you going to do it?" and turned his head away in disgust.

Sunday's town hall-style debate was a study in heated personal attacks and a stark reminder of the divisiveness of the 2016 race.

In a room that included former president Bill Clinton and three women who have accused him of sexual misconduct, Mr Trump threatened to jail his rival and lobbed incendiary allegations against her husband.

The 70-year-old real estate mogul apologised for "locker room talk" but accused Mr Clinton of being "abusive to women".

On Monday, Mr Trump doubled down on a pledge to investigate his rival if he wins, despite the suggestion being roundly denounced.

"Special prosecutor here we come," he sneered at a rally in Ambridge, Pennsylvania, branding Mr Clinton a "predator".

"If they want to release more tapes... we'll continue to talk about Bill and Hillary doing inappropriate things."