Democrats reached “Defcon One” on Thursday night, calling on Joe Biden to step aside after his disastrous debate performance.

Strategists and elected politicians are now openly considering replacing the US president as the Democratic nominee at this year’s convention, after fears about his mental capacity were realised during his head-to-head with Donald Trump.

Mr Biden gave a stuttering and incoherent performance at the debate, hosted by CNN in Atlanta, as he struggled to respond to questions about policy and often trailed off mid-sentence.

David Plouffe, who managed Barack Obama’s 2008 presidential campaign, told the network: “It’s kind of a ‘Defcon One’ moment”.

“The concern level is quite high. Biden is behind narrowly right now. He is the one who has to change the equation here and the biggest barrier that is keeping his ceiling too low is concerns about his age.

He added: “They are three years apart, but they seemed about 30 years apart tonight.”

David Axelrod, another Obama strategist, said: “I think you’re going to hear discussions that I don’t know will lead to anything but there are going to be discussions about whether he should continue.”

During one excruciating moment, Mr Biden lost his train of thought while discussing healthcare and tripped over his words before taking a protracted pause.

“We finally beat Medicare,” he began again, before a moderator cut him off. Trump chimed in: “He did beat Medicare. Beat it to death.”

Mr Biden spoke in a soft, raspy voice, which the White House attributed to a cold. In his two-minute pre-prepared statement at the end of the debate, he made several mistakes and was forced to correct himself.

Several Democrats anonymously briefed news outlets that Mr Biden’s position as the 2024 nominee now looked untenable.

“Biden is about to face a crescendo of calls to step aside,” one Biden-backing strategist told The New York Times. “Joe had a deep well of affection among Democrats. It has run dry.”

Another Democrat told NBC it was “time to talk about an open convention and a new Democratic nominee”.

A third told the Washington Post: “We’re so f—ed. He has great material. He just cannot deliver a single line.”

It would be challenging for the Democratic Party to replace Mr Biden at this stage in the campaign, after he won the party’s primary essentially unchallenged earlier this year.

Incumbent presidents running for a second term are almost never defenestrated by their party at their party’s convention, where the formal decision to award a nomination takes place.

To replace Mr Biden, the party would either require his consent, or for more than half of the state delegates who pledged support for him during the primary process to turn their backs on him.

If Mr Biden agreed to step aside, then the party would choose a new nominee in an open process of voting on the convention floor in Chicago in late August.

Politico reported on Thursday night that a strategist close to potential Democratic presidential candidates had been bombarded with messages requesting that their boss step up to challenge Mr Biden.

The shortlist to replace him includes Kamala Harris, his vice president, and Gavin Newsom, the California governor. Both said they strongly supported Mr Biden.

Ms Harris told CNN that Mr Biden had offered “a very clear contrast with Donald Trump on all the issues that matter to the American people”.

“Yes, there was a slow start, but it was a strong finish,” she said.

‘He’s our guy’

Mr Newsom, who acted as a “surrogate” for the Biden campaign, speaking to the media in Atlanta, flatly rejected calls for a replacement.

He said it was “farcical” to suggest that he could take over, telling reporters: “It’s a non sequitur. I don’t even understand the context of that.

“This is a president of the United States running for re-election. He’s our guy. And that’s part of the diversion tactic of the Right – to continue to sort of muddy the waters on all of that and that nonsensical speculation.”

When pressed on the issue in an interview with MSNBC, he added: “You don’t turn your back because of one performance. What kind of party does that?”

A poll taken by CNN after the debate showed that 67 per cent of viewers thought Donald Trump had won, compared with 55 per cent who thought he would win before it started.

Republicans at the debate were visibly delighted with Mr Biden’s performance, but said their campaign would not change if he was replaced.

“I don’t think it changes our approach at all,” Michael Whatley, the chairman of the Republican National Convention, told The Telegraph.

“This entire election cycle is about the policy solutions that President Trump is putting on the table. No Democrat is going to make a major change in policy, versus where Joe Biden is today.”