Joe Biden has attacked Donald Trump at a rally a day after the president’s “shocking” performance in a head-to-head debate.

“I don’t walk as easy as I used to, I don’t speak as smoothly as I used to, I don’t debate as well as I used to but I know what I do know,” said President Biden. “I know how to tell the truth.”

His performance in the debate against Trump on Thursday night was described as an “unmitigated disaster” by some in his own party after he paused and stumbled his way through, prompting calls for him to step down.

“I give you my word as a Biden, I wouldn’t be running again if I did not believe, with all my heart and soul, I can do this job,” he told the rally in North Carolina.

Mr Biden attacked the former president’s criminal record, calling Donald Trump a “one-man crimewave”.

“My guess is he set a new record for the most lies told in a single debate,” he added, telling the crowd he spent “90 minutes on the stage debating a guy with the morals of an alley cat”.

Hours later, Donald Trump was jubilant at a rally in Chesapeak, Virginia.

“Never mind that crooked Joe Biden spent a week at Camp David, resting, working, studying – he studied so hard he didn’t know what the hell he was doing,” said Mr Trump to the crowd.

“Biden’s problem is not his age, […] he’s got no problem other than his competence. He’s grossly incompetent,” he added.

Bad debate nights happen

Barack Obama tweeted his support for President Biden on Friday evening, saying: “Bad debate nights happen. Trust me, I know.”

But others remained unconvinced Joe Biden should run for president.

“I do not think President Joe Biden can be the Democratic Party’s standard-bearer in 2024,” former White House director of global engagement Brett Bruen told Sky News’ Yalda Hakim.

“Last night’s performance was astonishingly bad. You cannot just be strong on the teleprompter,” he said.

But Anthony Scaramucci, former White House communications director, blamed the president’s poor performance on his preparation.

“He wasn’t prepped right for that debate. He’s an older man,” Mr Scaramucci told Yalda Hakim.

“You don’t fill his head with facts and figures he’s never going to remember.”

There have been repeated calls for President Biden to step down.

It’s “time to talk about an open convention and a new Democratic nominee,” one Democratic politician told Sky’s US partner network NBC News.

Read more from Sky News:
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However, while Vice President Kamala Harris acknowledged that President Joe Biden had a “slow start” in the debate, she insisted that he finished “strong”.

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At a Waffle House restaurant in Atlanta after the debate, Biden told reporters he did not have concerns about his performance. “It’s hard to debate a liar,” he said.

Joe Biden’s stumbling performance in the US presidential debate has sparked alarm among Democrats – but could he be replaced against his will and how might that work?

A national US party has never tried to force an election candidate to step down in the modern era and their rules make it almost impossible to do so.

Latest: Reaction to Biden v Trump debate

The issue came before both parties in 2016, but neither took action and there’s currently no known effort to force Mr Biden to give way.

It would mean Democrat officials overturning the results of the state primaries – the votes that took place earlier this year that confirmed Mr Biden as the overwhelming pick for November’s election.

The 81-year-old won virtually all the delegates – and they are now set to go through the customary process of rubberstamping his nomination at the Democratic National Convention in August.

There are ways to replace a nominee if they die, resign or are incapacitated, but forcing Mr Biden out would mean delegates choosing another candidate.

It appears highly unlikely, as they were chosen in the primaries because of their loyalty to the president and with the trust that they would vote for him at the convention.

Read more:
‘Unmitigated disaster’ for Biden in TV debate with Trump
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However, there is a potential “loophole” in that Democrat rules allow them to “in all good conscience reflect the sentiments of those who elected them”.

So if concerns about his performance and mental acuity reach critical level this could be a way forward.

Watch a special programme with reaction to the US presidential debate on The World with Yalda Hakim on Sky News from 6pm

What if Biden withdraws himself?

This is the only plausible scenario for the Democrats to choose a new candidate to take on Donald Trump.

Mr Biden has repeatedly dismissed this option during other turbulent times, but if he changed his mind he could simply serve out the remaining months of his presidency.

Such a U-turn would create a frenzy among Democrats as there’s no protocol in place for him or the party to choose a new candidate before August’s convention.

Read more:
Excruciating Biden performance was among worst ever

It takes a majority of the party’s 4,000 or so delegates to win the presidential nomination – and Mr Biden secured about 3,900 of them in the primaries.

The president would have some influence over his pledged delegates, but ultimately they can vote as they please – so it could become a ‘free for all’ with candidates campaigning aggressively to win them over.

If the president decides to pull out after the convention takes place, the chair of the party can call a special meeting of around 500 members.

They can – in theory – choose a new nominee via a simple majority vote.

However, such a process would likely be far more turbulent with behind-the-scenes jockeying and public campaigning.

If Mr Biden withdrew even closer to the 5 November election, it could raise constitutional, legal and practical concerns.

For example, voting papers have to be printed well in advance and it might not be possible to change them in time.

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Would vice president Kamala Harris replace Biden?

If Mr Biden were to resign immediately as president, Kamala Harris would automatically succeed him – but would not also become Democratic nominee.

She might be politically favoured if he ditched his re-election bid, but Mr Biden’s delegates would not transfer to her as of right, and it would be a level playing field with the other hopefuls.

Who could be favourite if Biden steps back?

Two frontunners – if they chose to put themselves forward – could be California governor Gavin Newsom, 56, or Michigan governor Gretchen Whitmer.

They didn’t enter this year’s primaries but have both been talked about as potential Democrat nominees in future.

For now at least, Mr Newsom appears to be sticking firmly behind the president.

He told reporters after Thursday’s debate his party “could not be more wholly unified behind Biden” and he shouldn’t step aside.

Mr Newsom has been California governor since 2019 and was also San Francisco’s mayor for seven years. Before that, he founded a winery in the state’s upmarket Napa Valley.

Ms Whitmer, a lawyer and former prosecutor, was the Senate’s first female Democratic leader and became governor in 2018.

The 52-year-old also served as co-chair of Joe Biden’s 2020 election campaign.

She’s previously said she would have no interest in replacing Mr Biden in this year’s election – and backed him again on Friday.

“Joe Biden is running to serve the American people. Donald Trump is running to serve Donald Trump,” she said.

However, if the president pulls out of his own volition, could she change her mind?

Democrats including Minnesota’s Dean Phillips and Jason Palmer, who actually beat Mr Biden in American Samoa, might also try their hand.

They were his main rivals in the primaries but only got a couple of delegates each, so would likely struggle to pull in enough support.

Joe Biden and Donald Trump faced off in the first election debate of this year’s US presidential campaign on Thursday night.

The president was seen to mumble and stumble his way through, while Mr Trump repeated false claims that the 2020 election was rigged.

On this episode, Niall Paterson unpicks the debate between the senior citizens – with our US correspondent James Matthews in Georgia on the hits and misses, and pollster Scarlett Maguire on how American voters are feeling about the upcoming election.

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Producer: Soila Apparicio
Editor: Philly Beaumont