US President Joe Biden sought refuge on Sunday, appealing to Black voters at a church as he embarked on his campaign trail to rescue his reelection efforts and quell increasing calls for him to resign.

"We're all imperfect beings," the 81-year-old said to a receptive congregation in Philadelphia, where he was invigorated by worshippers chanting "four more years". Despite concerns from some Democratic lawmakers, analysts, and voters about his mental sharpness and physical stamina for a second term, sparked by a poor debate performance against Republican challenger Donald Trump last month, Biden has firmly stated he is fit to serve and the only one capable of defeating Trump.

On Sunday, he began a two-stop tour in the battleground state of Pennsylvania, followed by hosting the NATO leaders' summit in Washington later this week. So far, five Democratic lawmakers have urged Biden to withdraw, with the chorus of dissent gradually growing. Two prominent congressional Democrats on Sunday did not explicitly call for Biden to step down, but cautioned that he still needed to win over anxious 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." As some speculate about Biden passing the campaign to Vice-President Kamala Harris, Schiff said he believed she "could win overwhelmingly". Democratic Senator Chris Murphy, meanwhile, stated that "the president needs to do more", including unscripted events, to reassure voters.

Biden largely avoided addressing the current crisis during his seven-minute speech at Mt Airy Church of God in Christ, speaking to a constituency he has supported throughout his five-decade political career. "It's good to be home," he said to cheers, before lightly joking about his age. "I know I look like I'm only 40 years old but I've been around a little bit," he said.

Biden's campaign stops coincide with House Minority Leader Hakeem Jeffries scheduling a virtual meeting of senior Democratic representatives Sunday to discuss the best way forward, and Democratic Senator Mark Warner reportedly working to convene a similar forum in the Senate. With Washington in contemplation, First Lady Jill Biden is scheduled to campaign for her husband Monday in Georgia, Florida, and North Carolina, while the president prepares for the NATO summit beginning Tuesday.

Here, too, he will need 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, expressed admiration for Russian strongman Vladimir Putin, and claimed he could swiftly end the conflict in Ukraine. For now, Democratic heavyweights are largely suppressing any brewing discontent with their leader -- at least in public.

But 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 closely watched for any signs of more open rebellion. Meanwhile, for Biden and his team, the strategy appears to be to endure. The campaign has rolled out an intense battle plan for July, including a flood of TV spots and visits to all the key states.

That includes a visit to the US Southwest during the Republican convention July 15-18, at which Trump will be officially nominated as the party's presidential candidate.