Donald Trump launched into an expletive-fueled rant about Joe Biden in a leaked video of him on the golf course.

The former president claimed his 81-year-old rival has already quit in secret and that Vice President Kamala Harris will soon take his place as the Democrat nominee.

Trump referred to Biden as an "old, broken down pile of crap" who is about to "quit the race".

He said: "He just quit, you know - he's quitting the race. I got him out of the - and that means we have Kamala.

"I think she's gonna be better. She's so bad. She's so pathetic, she's just so f---ing bad."

The video comes as defiant President Viden vowed Wednesday to keep running for reelection, rejecting growing pressure from Democrats to withdraw after a disastrous debate performance raised questions about his readiness to keep campaigning, much less win in November.

But increasingly ominous signs were mounting for the president.

Two Democratic lawmakers have called on Biden to exit the race while a leading ally publicly suggested how the party might choose someone else.

And senior aides said they believed he might only have a matter of days to show he was up to the challenge before anxiety in the party boils over.

“Let me say this as clearly as I possibly can as simply and straightforward as I can: I am running … no one’s pushing me out," Biden said on a call with staffers from his reelection campaign. "I’m not leaving. I’m in this race to the end and we’re going to win.”

In his private conversations, Biden was focused on efforts to course correct from his rocky debate and on the threat that, in his view, Trump poses to the country, as he scoured for feedback on what went wrong last Thursday in Atlanta and took responsibility for his performance.

“We had a direct, open, clear-eyed conversation about the debate, his thoughts on what happened and why it wasn't his best evening or best debate,” Sen. Chris Coons, D-Del., who spoke with Biden on Tuesday, said in an interview with the Associated Press.

“He wanted advice. He was asking earnestly for input and comment on what he should do to restore confidence and support, and what's the best path forward.”

Coons, the president's closest ally on Capitol Hill, said Biden clearly understood the urgency, the difficulty and the importance of the election, as the senator advised that the president do more unscripted, open-ended events to restore confidence in his candidacy.

The two also spoke about Biden's schedule and its impact on his political efforts, particularly as he balances that task with critical governing tasks such as the NATO summit in Washington next week.

Biden's efforts to pull multiple levers to salvage his faltering reelection include his impromptu appearance with campaign aides, private conversations with senior lawmakers, a weekend blitz of travel and a network television interview. But he was confronting serious indications that support for him was rapidly eroding on Capitol Hill and among other allies.

Rep. Raúl Grijalva, D-Ariz., told The New York Times that though he backs Biden as long as he is a candidate, this “is an opportunity to look elsewhere” and what Biden "needs to do is shoulder the responsibility for keeping that seat — and part of that responsibility is to get out of this race.”