US President Joe Biden has said only the "Lord Almighty" could convince him to end his bid for re-election, as he sat for a rare primetime interview in an effort to calm Democratic concern over his candidacy.

Speaking to ABC News on Friday, Mr Biden also declined to take a cognitive test and make the results public in order to reassure voters he is fit to serve another term.

"I have a cognitive test every single day. Every day I have that test - everything I do [is a test]," he told George Stephanopoulos.

The 81-year-old once again pushed back on the idea, aired by some Democratic officials and donors, that he should stand aside for a younger alternative following his disastrous debate with Donald Trump last week.

Throughout the interview, Mr Stephanopoulos pressed the president on his capacity to serve another term, asking Mr Biden if he was in denial about his health and ability to win.

“I don’t think anybody’s more qualified to be president or win this race than me," Mr Biden said, blaming his poor performance last week on exhaustion and a "bad cold". In the 22-minute interview, he also:

• Attempted to ease Democratic fears he had lost ground to Donald Trump since the debate, saying pollsters he had spoken to said the race was a "toss-up"
• Rejected suggestions allies may ask him to stand aside. “It’s not going to happen," he said
• Dismissed repeated questions about what would compel him to leave the race. "If the Lord Almighty came down and said, ‘Joe, get out of the race,’ I’d get out of the race,” he said. “The Lord Almighty’s not coming down”

The president answered questions more clearly than he did on the debate stage last week, but his voice again sounded weak and occasionally hoarse.

It was a sharp contrast to his performance at a rally in Madison, Wisconsin, on Friday, where an energised Mr Biden 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.

The interview and the rally come at a critical moment for his campaign, with donors and Democratic allies considering whether to stick with him.

The campaign is aware 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.

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.

Some major Democratic donors have begun to push for Mr Biden to step down as the party's nominee, publicly warning they will withhold funds unless he is replaced.

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.

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

On Friday, reports emerged that House minority leader Hakeem Jeffries had scheduled a Sunday meeting with senior House Democrats to discuss Mr Biden’s candidacy.

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

“President Biden has done enormous service to our country, but now is the time for him to follow in one of our founding father, George Washington’s footsteps and step aside to let new leaders rise up 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.

Speaking to reporters later on Friday, Mr Biden said he understood that Mr Warner "is the only one considering that" and that no one else had called for him to step down.

The same day, Massachusetts Governor Maura Healey, a Democrat and ally of Mr Biden, issued a statement urging the president to "carefully evaluate" whether he remains the Democratic nominee.

"Whatever President Biden decides, I am committed to doing everything in my power to defeat Donald Trump,” she said.

Some Democratic voters, too, have lost faith in Mr Biden's capacity to run. In a Wall Street Journal poll released on Friday, 86% of Democrats said they would support Mr Biden, down from 93% in February.

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.