Trump: Paul Ryan does not support me because he wants to be president

US presidential candidate Donald Trump has once again criticised one of the main detractors from within his own Republican party, accusing House Speaker Paul Ryan of withdrawing his support because of his own presidential ambitions.

Ryan, the GOP's highest elected official, has not yet officially backed out from Trump's campaign, but said he will not be defending him or actively campaigning any longer.

In an interview with Good Morning America on Tuesday, Trump made the claim that Ryan wanted to run for the nomination in 2020 and that he would not want to face the New York businessman.

He was asked by the interviewer whether he thought Ryan wanted him to win or not.

"Well, maybe not, because maybe he wants to run in four years or maybe he doesn't know how to win," Trump said. "I mean, who can really know. But I know I'm in his territory and they are all screaming for Trump."

Ryan changed his position after the furore which enveloped Trump following the release of a leaked tape which appeared to show the Republican nominee bragging about sexual misconduct.

Trump has since responded in his own unique style, repeatedly attacking Ryan for "weak and ineffective" leadership via Twitter.

Democrat Hillary Clinton appears to be building a sizeable lead in the majority of national polls ahead of the final televised debate on Wednesday.

Bookmaker Paddy Power has already begun to pay out on a Clinton victory, confident that the tycoon's self-implosion has ended his chances of victory.