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 Mr Ryan and other establishment Republicans had abandoned him.

Key points: Donald Trump slams Paul Ryan for inspiring party disloyalty, shames "foul-mouthed" John McCain for dropping him over "locker room remarks"

Donald Trump slams Paul Ryan for inspiring party disloyalty, shames "foul-mouthed" John McCain for dropping him over "locker room remarks" Barack Obama calls on senior Republicans who have criticised latest comments to disendorse Mr Trump

Barack Obama calls on senior Republicans who have criticised latest comments to disendorse Mr Trump Republicans are increasingly worried Mr Trump could hurt their Congress majorities

The day after Mr Ryan, the top Republican in Congress, told party politicians he was breaking with the presidential nominee and would not campaign for him, Mr 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," Mr Trump said in a tweet on Tuesday.

"The very foul-mouthed Sen. John McCain begged for my support during his primary (I gave, he won), then dropped me over locker room remarks!" he wrote in another tweet.

Loading

Mr Trump complained in another tweet that it was hard to do well with "zero support" from Mr Ryan and others, but added later:

"It is so nice that the shackles have been taken off me and I can now fight for America the way I want to."

He later said in an interview with Fox News host Bill O'Reilly that he neither wanted nor needed Mr Ryan's support.

Mr 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 Mr Ryan said he would not withdraw his endorsement of the New York businessman.

A Reuters/Ipsos poll of likely voters, released today, found 58 per cent of Republicans wanted Mr Trump to stay atop their party's ticket and 68 per cent said the Republican leadership should stand by him.

Obama calls on Republicans to disendorse Trump

Meanwhile, US President Barack Obama used a Clinton campaign rally in Greensboro, North Carolina, to urge senior Republicans to disendorse Mr Trump.

"Now you find a situation in which the guy says stuff that nobody would find tolerable if they were applying for a job at 7-Eleven," he said.

"There are still a number of elected Republican officials, some of whom I know and I'm sure are embarrassed and say 'wow that was a really terrible thing he said', but they can't bring themselves to say 'I can't endorse this guy'.

"You can't repeatedly denounce what is said by someone and then say, 'but I'm still going to endorse him to be the most powerful person on the planet' and put them in charge."

Barack Obama denounced Republican leaders who had criticised remarks made by Donald Trump but still supported him. ( Reuters: Carlos Barria )

Republican worries rise as campaign heads to Florida

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

Sorry, this video has expired Republicans struggle to contain Donald Trump's scandal-struck campaign.

Many Republicans are worried Mr Trump's 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 Mr Trump's lewd remarks on the video, and roughly one in 10 have called for him to drop out of the race.

Mr Trump's vice presidential running mate, Indiana Governor Mike Pence, reinforced his support in television interviews on Monday.

AP/Reuters