United States President Joe Biden is facing growing pressure from within his party to prove he is physically and mentally fit for office, with a Democratic lawmaker publicly calling on him to end his re-election bid for the first time.

Biden’s candidacy has been under a cloud since a disastrous debate performance against Republican challenger Donald Trump that saw the 81-year-old Democrat stumble over his words and lose his train of thought.

On Tuesday, Lloyd Doggett, a House Representative from Texas, became the first member of his party to publicly call on Biden to quit the race.

“I represent the heart of a congressional district once represented by Lyndon Johnson. Under very different circumstances, he made the painful decision to withdraw,” Doggett said in his statement.

“President Biden should do the same.”

Marie Gluesenkamp Perez, a House Representative for Washington state, stopped short of calling on Biden to withdraw, but said she believed Thursday’s debate performance would cost him the election in November.

“We all saw what we saw, you can’t undo that, and the truth, I think, is that Biden is going to lose to Trump. I know that’s difficult, but I think the damage has been done by that debate,” Perez said in an interview with the KATU news channel in Portland, Oregon.

Jared Golden, a House Representative in Maine, also said that he believed that Trump would win and he was “OK with that”.

“Lots of Democrats are panicking about whether President Joe Biden should step down as the party’s nominee,” Golden said in an opinion piece published in The Bangor Daily News.

“Biden’s poor performance in the debate was not a surprise.”

Former House Speaker Nancy Pelosi and Representative Jim Clyburn also added their voices to those scrutinising Biden’s condition, saying it was legitimate to raise concerns about his health following the debate.

“I think it’s a legitimate question to say, is this an episode, or is this a condition? And so, when people ask that question, it’s completely legitimate – of both candidates,” Pelosi said in an interview with MSNBC.

While Democratic insiders have been privately raising concerns about Biden’s fitness with media outlets for days, the series of public comments intensifies pressure on the president to assuage growing doubts about his electability.

The White House said on Tuesday that Biden would hold a series of meetings and appearances to quash concerns about his fitness, including a news conference and his first sit-down television interview since May.

The White House press secretary, Karine Jean-Pierre, told a media briefing that Biden was sick with a cold during the debate and had “a bad night”.

“We really, truly want to turn the page on this,” Jean-Pierre told reporters.

“We really want to be able to get out there and speak directly to the American people.”

During a fundraising event later on Tuesday, Biden blamed his poor performance on back-to-back trips to France and Italy, although he spent the week leading up to the debate behind closed doors at presidential retreat Camp David.

“I wasn’t very smart. I decided to travel around the world a couple of times,” Biden said.

Biden added that he did not listen to his advisers about his travel schedule and joked that he “almost fell asleep on stage” during the debate.

In a CNN poll published after the debate, three-quarters of registered voters said that Democrats would have a better chance at winning the election with someone other than Biden on the ticket.

Voters also favoured Trump over Biden, 49 percent to 43 percent.

Vice President Kamala Harris did moderately better, gaining the support of 45 percent of voters compared to Trump’s 47 percent.

Other Democrats floated as potential replacements, including California Governor Gavin Newsom and Transportation Secretary Pete Buttigieg, trailed Trump by similar margins as Biden.

Harris on Tuesday pushed back on the suggestion that Biden should step aside.

“Look, Joe Biden is our nominee. We beat Trump once and we’re going to beat him again, period,” she said in an interview with CBS News.