US President Joe Biden vowed to stay the course in his re-election bid and defeat Donald Trump in a defiant speech on Friday, as questions continue to swirl over whether he will drop out of the race.

At a rally in Madison, Wisconsin, the 81-year-old acknowledged his disastrous performance in last week's CNN debate. "Ever since then, there’s been a lot of speculation. What’s Joe going to do?” he told the crowd.

“Here’s my answer. I am running and going to win again,” Mr Biden said, as supporters in the crucial battleground state cheered his name. It marked his latest commitment to staying in the race as he seeks to defuse a political crisis that has snowballed in recent days.

The 17-minute speech, which was more energetic than his widely panned performance on the debate stage, comes at a critical moment for his campaign, with donors and Democratic allies considering whether to stick with him.

The campaign recognises that the next few days could make or break his re-election bid, according to various reports in US media, as Mr Biden seeks to regain ground that he lost to his Republican rival Donald Trump following the debate.

After his speech in Madison, a rare sit-down interview will air on ABC News at 20:00 EDT which will also be closely watched by those questioning his commitment and fitness.

As he took the stage at the rally, Mr Biden passed one voter who was holding a sign reading "Pass the torch, Joe". Another voter who stood outside the venue held a sign that read "Save your legacy, drop out!".

"I see all these stories that say I'm too old," Mr Biden said at the rally, before triumphing his record in the White House. "Was I too old to create 15 million jobs?" he said. "Was I too old to erase student debt for five million Americans?"

“Do you think I’m too old to beat Donald Trump?" he asked, as the crowd responded "no".

Referencing Trump's criminal conviction in New York, and the other charges he is facing in separate cases, he called his rival a "one-man crime wave".

Pressure on Mr Biden to step aside has only grown following the debate which was marked by several instances where he lost his train of thought, raising concerns about his age and mental fitness.

His campaign is planning an aggressive come-back. His wife, Jill Biden, as well as Vice-President Kamala Harris, are planning a campaign blitz to travel to every battleground swing state this month.

Mr Biden, who is due to speak at another rally in Pennsylvania on Sunday, thanked the vice president for her support. She has emerged as the most likely candidate to replace him on the Democratic ticket if he were to step down.

On Thursday, Mr Biden acknowledged that he "screwed up" in the debate. He has blamed jet lag for his poor performance, saying that his busy travel schedule prevented him from getting sufficient rest before the debate. "I didn’t listen to my staff... and then I nearly fell asleep on stage,” he said.

The Washington Post has reported that Mr Biden's senior team are aware of the pressure coming from within the Democratic Party to decide on the future of his candidacy within the next week.

Three Democrats in the House of Representatives in Congress have now called for him to withdraw from the race - Lloyd Doggett of Texas and Raúl Grijalva of Arizona and Seth Boulton of Massachusetts.

“President Biden has done enormous service to our country, but now is the time for him to follow in one of our founding fathers, George Washington’s footsteps and step aside to let new leaders rise and run against Donald Trump,” Mr Moulton told radio station WBUR on Thursday.

However, no senior Democrats have called on him to quit, as his campaign has pointed out to reporters.

On Friday, reports emerged that Senator Mark Warner was attempting to form a group of fellow Democratic senators to ask Mr Biden to drop out of the race. The reports, including one in the Washington Post, suggested Mr Warner had deep concerns following the CNN debate.

At the rally in Madison, multiple Biden supporters told BBC News that they supported his bid for re-election and were not concerned about the debate debacle.

"I’m not worried about his health. I think he can go all the way to the election and beyond," said primary school teacher Susan Shotliff, 56.

Some said that while Mr Biden struggled for words, more focus should be on his Republican rival. "During the debate, [Trump] told a bunch of lies. How is that any worse than what Biden did?" said Greg Hovel, 67.

Others expressed more concern. "I wanted to have a first-hand look at how he’s like, his mannerisms, his energy," said Thomas Leffler, a health researcher from Madison. "I’m worried about his capacity to beat Trump."

"As he gets older, I think it’s going to increasingly be an issue. But I’ll vote blue no matter what," he said.