Philadelphia, Pennsylvania - Embattled President Joe Biden sought out safe ground Sunday, courting Black voters at a church as he hit the campaign trail to salvage his reelection bid and silence growing calls to quit.

"We're all imperfect beings," the 81-year-old told a welcoming congregation in Philadelphia, where he received a jolt of rejuvenation from worshippers who chanted "four more years."

Biden has dug in despite an uprising among some Democratic lawmakers, 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.

But the president has unequivocally declared he is fit to serve and is staying in the race.

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On Sunday, he embarked on a two-stop swing in the battleground state of Pennsylvania, before he hosts the NATO leaders' summit in Washington later this week.

So far, five Democratic lawmakers have called on Biden to drop out, with the drumbeat of dissent slowly rising.

Biden himself largely avoided discussing the crisis at hand 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.

Biden's campaign stops come as House Minority Leader Hakeem Jeffries scheduled a virtual meeting of senior Democratic representatives Sunday to discuss the best way forward, and Democratic Senator Mark Warner is reportedly working to convene a similar forum in the Senate.

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 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.

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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.

For now, Democratic heavyweights are largely keeping a lid on any simmering 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 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 all the key states.

That includes a visit to the US Southwest during the Republican convention on July 15-18, where Trump will be anointed the party's official presidential nominee.