Embattled US President Joe Biden hit the campaign trail Sunday seeking to reassure voters he was up for a grueling reelection fight, even as more top Democrats reportedly joined the chorus urging him to quit the race.

The 81-year-old remained defiant as he hop-scotched across Pennsylvania -- first to the relative safe ground of a predominantly Black church and a campaign field office, and then to a rally with union workers.

As Biden arrived in the state capital Harrisburg, reporters shouted questions including whether the Democratic Party was behind him, to which the president grinned and loudly responded: "Yes."

But an uprising has swelled among some fellow Democrats, analysts and voters concerned he lacks the mental acuity and physical fitness to serve a second term -- worries brought to the fore by a disastrous debate performance last month against Republican challenger Donald Trump.

So far, five Democratic lawmakers have publicly called on Biden to drop out.

The drip-drip of dissent increased Sunday as four senior congressmen, including ranking House Judiciary Committee Democrat Jerrold Nadler and ranking Armed Services Committee Democrat Adam Smith, said on a call with senior party lawmakers that it was time for Biden to exit the race, according to The New York Times and Politico, which cited people familiar with the call.

The president, however, has unequivocally declared he is fit to serve, saying that he is the only one who can defeat Trump, and that he is staying in the race.

Democratic lawmakers will return to Capitol Hill Monday under pressure to either fall in line behind Biden or urge him to step aside.

Reported attempts by Senator Mark Warner to assemble his colleagues to address the crisis were called off, US media reported Sunday, with senators expected at their regular caucus meeting on Tuesday instead.

As for the president, his next major test will come on Thursday, when he is scheduled to hold a press conference during the NATO leaders' summit in Washington.

Two high-profile congressional Democrats on Sunday stopped short of calling for Biden to quit, but warned he still needed to win over worried voters.

"There's only one reason" the race between Trump and Biden "is close, and that's the president's age," Representative Adam Schiff told NBC's "Meet the Press."

Democratic Senator Chris Murphy said "the president needs to do more," including unscripted events, to reassure voters.

"This week is going to be absolutely critical," Murphy told CNN.

Biden himself largely avoided discussing the crisis when he gave a seven-minute address at Mt. Airy Church of God in Christ, speaking to a constituency he has embraced throughout his half-century political career.

"It's good to be home," he said to cheers, before joking briefly about his age.

"I know I look like I'm only 40 years old but I've been around a little bit," he said.

- Democrats debate -

Afterwards he visited a Democratic campaign field office to meet and thank staff. He spoke for a few minutes, without using a teleprompter or notes.

Some Democrats remain squarely behind Biden, including Senator John Fetterman of Pennsylvania, who accompanied him on the campaign excursion.

"There's only one person in this country who has kicked Trump's ass in an election, and that is your president," Fetterman told the Democratic staffers in Philadelphia, as Biden looked on.

With Washington ruminating, First Lady Jill Biden is scheduled to campaign for her husband Monday in Georgia, Florida and North Carolina, while the president preps for the NATO leaders' summit, beginning Tuesday.

Here, too, he will find himself having to reassure allies at a time when many European countries fear a Trump victory in November.

The 78-year-old Republican has long criticized the defense alliance, voiced admiration for Russian strongman Vladimir Putin, and insisted he could bring about a quick end to the fighting in Ukraine.

With election day just four months away, any move to replace Biden as the nominee would need to be made sooner rather than later, and the party will be scrutinized for any signs of more open rebellion.

Meanwhile, for Biden and his team, the strategy seems to be to ride it out.

The campaign has unveiled an intense battle plan for July, including an avalanche of TV spots and trips to key states.

© Agence France-Presse