WASHINGTON - Embattled presidential hopeful Donald Trump lashed out against "disloyal" Republicans in a ranting tweetstorm on Tuesday, deepening a split that threatens the future of the 162-year-old party.

Declaring himself unchained from party strictures, the White House nominee berated Republican leaders for offering "zero support" and promised a bareknuckle 28-day campaign against Hillary Clinton.

Trump called Paul Ryan, the nation’s top elected Republican, a "weak and ineffective leader".

He ridiculed the party’s 2008 White House nominee John McCain as "very foul mouthed".

"It is so nice that the shackles have been taken off me and I can now fight for America the way I want to," Trump said in a three-hour cyber outburst.

"I will teach them!"

He doubled down in an interview with Fox News on Tuesday night, saying of Ryan: "I don’t want his support. I don’t care about his support."

"I wouldn’t want to be in a foxhole with these people, including Ryan, especially Ryan," he added later in the interview, referring to Republicans who have turned their backs on him.

Relations between Trump and party leaders have always been strained – from the outset many considered the bombastic reality-TV-star-turned-politico unfit to take up the mantle of Abraham Lincoln and Ronald Reagan.

But the snapping point has been Trump’s precipitous fall in the polls after the emergence of a video in which he bragged about groping women.

US President Barack Obama addressed the scandal on Tuesday, after White House spokesman Josh Earnest earlier in the day called the comments "repugnant" and said most people would consider the actions described as "sexual assault".

"You don’t have to be a husband or a father to say that’s not right. You just have to be a decent human being," Obama said while campaigning for Clinton in North Carolina.

Trump has apologised for the comments, saying they were just "locker-room" banter.

"If that’s why I’m going to lose an election... that’s going to be sad," he said in the Fox interview.

He added with his typical bombast: "But I think we’re going to win."

The latest Real Clear Politics polling average has Clinton ahead by six points across the country, and the latest polls show her ahead in a slew of must-win states for Trump.

With his campaign in a tailspin, Trump seemed determined to ram a wedge between party leaders and the radicalized grassroots that propelled his political career. - AFP