Among growing ranks of Democrats one thing is certain: President Joe Biden cannot continue as their party’s presumptive nominee or they will lose to Donald Trump in November. Much less certain is this: how to remove him.

On Tuesday, a president known for his doggedness offered a reminder of how difficult it may be to dislodge him as he and his team unleashed a desperate campaign — on the airwaves and behind the scenes — to claw back support from some wavering Democrats in Congress.

To the extent that they succeeded they have only deepened the agony that has afflicted many Democrats since the president’s catastrophic performance in a debate against Trump nearly two weeks ago.

In the immediate aftermath, many Biden detractors consoled themselves that the president and his team would soon bow to what they viewed as the inevitable — nudged along by a steady drip of negative polling results and television medical diagnoses — and step aside. Now, the dawning realisation that the president is digging in is instilling a swirl of confusion, despair and anger in the party.

“This isn’t just about whether the president is fit to serve or run. The fear among everyone I’m talking to is that Biden’s intransigence will drag down close races in the House and Senate and imperil democracy itself,” said Gideon Stein, an entrepreneur and prominent Democratic donor, explaining the existential fear that party members attach to another Trump presidency.

Another prominent donor accused Biden and his team of “playing Russian roulette with the world at stake” while a former senior member of the Democratic National Committee described the president’s behaviour as that of “a mad king”. 

Inside the Biden campaign, some have succumbed to the same foreboding. At least one senior staffer has told friends in recent days that they believed the endeavour was now doomed, in spite of the president’s defiant public statements. 

Following the debate, there were reports of Biden family members blaming longtime aides for the president’s poor performance, as well as bickering between the campaign staff and the White House over the president’s travel schedule. 

Meanwhile, talk about the spoils of victory — ambassadorships and top posts in the administration — has given way to worries about being accused of a cover-up to conceal from the public the extent of Biden’s decline.

“Now that things have deteriorated they are concerned that crazy Republicans will try to weaponise things and attack them in as many ways as possible,” said a person who spoke directly with three Biden campaign operatives. Discussions about whether they should remain with the campaign or jump ship were now active, this person added.

Kevin Munoz, a spokesperson for the Biden campaign, said it was “working really hard because on winning campaigns you work really hard”. He added: “There’s an immense sense of pride across our office, because we know how important and critical that work we are doing here is for the fate of our democracy.”

In recent days, the Biden team has largely overlooked Trump to focus its attention on Capitol Hill, where the president and his team have been fighting to prevent further defections by their party brethren.

On Tuesday, some well-known representatives emerged from a two-hour meeting of the Democratic caucus to pledge their allegiance — from the youthful Alexandria Ocasio-Cortez to the veteran Jerrold Nadler, both of New York.

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“The president made very clear yesterday that he’s running. For me that’s dispositive; we have to support him,” Nadler said.

Two days earlier, on a conference call convened by Hakeem Jeffries, the Democratic House leader, Nadler was one of seven senior representatives to call for the president to end his campaign, according to a fellow lawmaker.

Nadler did not comment on his apparent change of mind, only saying that Biden’s record was “excellent” and Trump would be a “danger to democracy”.

The Tuesday morning gathering was variously described by attendees as “sombre” and “funereal”. One indication of the party’s current mistrust was that members of Congress were not allowed to bring their phones inside.

Maxwell Frost, a 27-year-old first-term representative from Florida, said he drew encouragement from a discussion with Anita Dunn, one of the president’s top aides and most seasoned Washington insiders, after Biden talked to the Congressional Black Caucus on Monday night. 

“She made me feel really good,” said Frost, noting that the campaign would soon be launching a $50mn ad blitz and had plans to unleash Biden on the road. They discussed “how do we make sure that this moment doesn’t linger for a long time? How do we make sure we come back from it? Because we did take a hit from it in some of these battleground states.”

But others battened down the hatches, preferring not to show their hands. Meanwhile, in the shadows there was much talk about letters being drafted by various heavyweights, calling for the president to step aside.

 That seemed unlikely before Thursday, at the end of this week’s Nato summit in Washington, when the president will take part in what the White House has called “a big boy press conference”. Biden will face the press corps without the safety of a teleprompter: another high-stakes test of his fitness for office.

Already, though, some were bracing for an inconclusive result that would leave the party in the same predicament: Biden not performing well enough to allay concerns about his fitness for office — but not so disastrously that the case for his removal would become urgent and irrefutable.

“A lot of people told ourselves he could manage it and that it wasn’t that bad,” a Democratic lobbyist said, reflecting on the past 12 days. “Now it’s hard to say that.”