White House candidate Donald Trump has stepped up his attacks against US House Speaker Paul Ryan, calling him a weak and ineffective leader but saying his campaign "shackles" were off now that Ryan and other establishment Republicans have abandoned him.

The day after Ryan, the top Republican in Congress, told party MPs he was breaking with the presidential nominee and would not campaign for him, Trump issued a barrage of social media posts criticising Republicans who have fled his campaign.

The stinging attacks deepened a dramatic rift in the party over the former reality TV star, who has seen a string of Republican defections after a video surfaced on Friday showing him bragging crudely to a reporter in 2005 about groping women and making unwanted sexual advances.

"Our very weak and ineffective leader, Paul Ryan, had a bad conference call where his members went wild at his disloyalty," Trump said in a tweet on Tuesday.

He complained in another tweet that it was hard to do well with "zero support" from Ryan and others, but added in a later Twitter post, "It is so nice that the shackles have been taken off me and I can now fight for America the way I want to."

Ryan told congressional Republicans he would put his energy into preserving party majorities in Congress, all but conceding that Democratic candidate Hillary Clinton would likely win the White House in the November 8 election.

The move angered some Trump supporters, although Ryan said he would not withdraw his endorsement of the New York businessman.

Trump, 70, has portrayed himself as tough on national security, and his campaign released a television advertisement on Tuesday featuring footage of Clinton, a 68-year-old former secretary of state, stumbling last month after leaving a service commemorating the September 11 attacks.

"Hillary Clinton doesn't have the fortitude, strength or stamina to lead in our world," the ad's narrator says.

"She failed as secretary of state. Don't let her fail us again."

Trump, whose campaign has been marked for months by controversies over both his policies and brash style, has slipped further behind Clinton in opinion polls.

Many Republicans are worried his chaotic campaign could hurt their chances of holding majorities in the House of Representatives and Senate in next month's election, and will inflict long-term damage on the party.

In an extraordinary party revolt, nearly half of all 331 incumbent Republican senators, House members and governors have condemned Trump's lewd remarks on the video, and roughly one in 10 have called for him to drop out of the race, a Reuters review of official statements and local news coverage indicates.

The Reuters/Ipsos State of the Nation project released on Monday estimated that Clinton had at least a 95 per cent chance of winning the 270 Electoral College votes needed to become president.

The polling did not capture reaction to Trump's performance in Sunday's debate or Friday's news reports on the video.