US President Barack Obama has called on Republicans to join House Speaker Paul Ryan and withdraw their support for Donald Trump.

Speaking at a rally in Greensboro, North Carolina, Obama delivered a damning indictment of Republicans who continue to support their presidential candidate.

Trump has faced fierce criticism after a video tape emerged last week, from 2005, in which he talked in vulgar terms about making unwanted sexual advances towards women.

Obama said: "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.

"The fact that now you've got people saying, 'Well, we strongly disapprove. We really disagree. We find those comments disgusting. But we're still endorsing him. We still think he should be president.' That doesn't make sense to me", he added.

Several minutes after Obama began speaking, two protesters - a young man and woman - moved towards the stage and revealed T-shirts that said "Bill Clinton Rapist".

The move, allegedly by pro-Trump supporters, was in reference to the three women who accuse President Bill Clinton of committing sexual crimes against them. Clinton was never charged in those cases.

Obama joked: "This is the great thing about politics in America. It takes all kinds. This is our democracy at work. This is great", before they were led out by security.

A protester is led away during Barack Obama's speech. Credit: Reuters

Meanwhile Trump has slammed the Republicans who dismiss his case for the White House as disloyal.

Speaking on The O'Reilly Factor show on Tuesday, he called Paul Ryan "weak and ineffective", and would be better off without his support.

He added: "I don't want his support. I don't care about his support."

Senator John McCain was the latest in a long line to withdrawn their backing for Trump, but when asked on his views, he said McCain has the "dirtiest mouth in all of the Senate", before adding they will both return when he is elected president.

Donald Trump addressed a crowd in Panama City, Florida, on Tuesday. Credit: Reuters

At the same time as Mr Trump was speaking, Hillary Clinton's campaign chairman, John Podesta, said the FBI would be investigating claims his emails were hacked after several were released by Wikileaks.

Podesta has blamed the Russian government, who he suggested were colluding with Trump in a bid to "interfere with the US election process".

His claims come amid a wider probe into to the hacking of Democrat websites.