Paul Ryan, the top Republican in the US Congress, has taken the extraordinary step of distancing himself from Donald Trump, risking a backlash from lawmakers and deepening a crisis over his party's struggling presidential nominee.

In a conference call with congressional Republicans on Monday, Ryan all but conceded that Democrat Hillary Clinton will likely win the White House on November 8, saying he'll put his full energy into preserving Republican majorities in Congress so as not to give her a "blank check".

Ryan, the Speaker of the House of Representatives, said he won't defend Trump or campaign for him after the uproar over the New York businessman's sexually aggressive comments that surfaced on Friday.

Ryan's announcement added to the party's worst turmoil in decades and reinforced the growing sense of isolation around Trump, who has never previously run for public office.

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Clinton has led Trump in most national opinion polls for months and Trump's poll numbers slipped further after the emergence on Friday of a 2005 video showing the former reality TV star bragging about groping women and making unwanted sexual advances.

It didn't take long for Trump to hit back at Ryan.

"Paul Ryan should spend more time on balancing the budget, jobs and illegal immigration and not waste his time on fighting Republican nominee," Trump wrote on Twitter.

Ryan, who had expressed disgust over the tape and cancelled a campaign event with Trump at the weekend, did not completely cut ties to Trump, later clarifying on the conference call he was not withdrawing his endorsement.

Many Republican members of Congress are concerned that Trump's chaotic campaign could ruin their chances of holding their majorities in the House and Senate and could inflict long-term damage to the party.

According to Reuters, nearly half of all 331 incumbent Republican senators, Congress members and governors have condemned Trump's remarks, and roughly one in 10 have called on him to drop out of the race.