An embattled US President Joe Biden faced escalating pressure on Sunday from fellow Democrats worried about his candidacy, concerns he aimed to ease with campaign stops in the battleground state of Pennsylvania.

Biden, 81, has faced growing calls to end his reelection bid after a halting performance in a June 27 debate with Republican Donald Trump, 78, raised questions about his ability to do the job for another four years. He has vowed to stay in the race, dismissing calls for him to drop out as "nonsense" in a fundraising email on Saturday.

On Sunday, the Democratic president attended a Black church service in Philadelphia and was later due to travel to the state capital, Harrisburg, for an event with union members and local Democrats.

The coming week is crucial, Democratic Senator Chris Murphy said on CNN's "State of the Union." He encouraged the president to try hold a town hall or news conference to convince voters he is "the old Joe Biden."

"I do think the president needs to do more," Murphy said. "I do think the clock is ticking."

Democrats also suggested that Vice President Kamala Harris, seen as the likeliest candidate to replace Biden in the November 5 election were he to bow out, could perform well.

A much-anticipated Biden interview with ABC News on Friday did little to ease concerns of lawmakers and donors.

Democratic US Representative Adam Schiff said on NBC's "Meet the Press" that Biden needs to move swiftly to put concerns to rest.

"I think she very well could win overwhelmingly, but before we get into a decision about who else it should be, the president needs to make a decision about whether it's him," Schiff said of Harris, a fellow Californian.

House Democratic Leader Hakeem Jeffries scheduled a meeting on Sunday with senior House Democrats to discuss the president's candidacy.

A Democratic National Committee (DNC) member from Florida, Alan Clendenin, urged Biden to step aside on Sunday.

"Joe Biden will be remembered by historians as one of the finest presidents in American history, but this election is about the next four years, not the last three and a half," Clendenin said.

The DNC has steadfastly supported Biden since his debate stumbles so any defections would suggest a further deepening of the crisis.

In Friday's interview, Biden said only the "Lord Almighty" could persuade him to drop out, dismissing the prospect that Democratic leaders could join forces to try to talk him into standing down.

Pennsylvania is one of the half dozen or so states that can swing Democratic or Republican and are expected to determine the outcome of what has been a tight race.

Sunday's trip - Biden's 10th to Pennsylvania during the 2024 election cycle - is part of a July voter outreach blitz by the Democratic Party that includes a US$50 million media campaign aimed at events such as the Olympics and travel by the president, the first lady, Harris and her husband to every battleground state.

The campaign said it aims to have volunteers and staff knock on more than 3 million voters' doors in July and August.

Pressure from Congress is expected to steadily increase in the coming days as lawmakers return to Washington from a holiday recess, and donors mull their willingness to keep funding Biden's campaign.

The political crisis is unfolding as Biden prepares to host dozens of world leaders at a high-stakes Nato summit in Washington and hold a rare solo news conference.

Five US lawmakers have called for Biden to end his reelection bid, including Representative Angie Craig of Minnesota, the first Democratic member of the House of Representatives from a battleground district, with others said poised to join in.

“Given what I saw and heard from the President during last week’s debate in Atlanta, coupled with the lack of a forceful response from the President himself following that debate, I do not believe that the President can effectively campaign and win against Donald Trump,” Craig, a top 2024 target of House Republican efforts, posted on X.

Two letters are circulating among House Democrats calling for Biden to step aside, House Democratic sources have said.

US Senator Mark Warner of Virginia invited fellow senators to a meeting on Monday to discuss Biden's campaign. Biden told reporters he had spoken with 20 congressional Democrats, who urged him to stay on. Biden said he knew of no senators ready to join Warner.

Senator Bernie Sanders, 82, who has run for the Democratic nomination for president in the past, stood firmly in Biden's camp on Sunday, saying Democrats' focus should be on policy.

"This is not a beauty contest," he said on CBS' "Face the Nation." (Reuters)