But he also listened to the concerns and feedback, say two people who spoke about private conversations on condition of anonymity.

In the appeal, the president promised to campaign harder in the future, to travel more often and to convey his message to voters more directly.

He will have the opportunity to attend a service at the Mount Airy Church of God in Christ in Northwest Philadelphia and then travel to the state capital of Harrisburg to speak at an event with union members and local Democrats.

US President Joe Biden says only the “almighty Lord” can convince him to give up his re-election campaign. (AP PHOTO)

On Sunday evening, Biden returns to Washington, where NATO leaders will meet starting Tuesday for a three-day summit to mark the 75th anniversary of the military alliance.

While the focus may shift more to Russia's war in Ukraine than to questions about Biden's re-election campaign, the political situation of the 81-year-old Biden remains precarious.

Five Democratic lawmakers called on him to give up his candidacy for re-election before November, and more could follow suit in the coming days when Congress reconvenes.

The face-to-face meeting will give congressional Democrats an opportunity to discuss their concerns about Biden's ability to make it through the remaining four months of the campaign – not to mention four more years in the White House – and their real prospects of beating former Republican President Donald Trump.

The president's campaign has announced a $50 million ($74 million Australian) month-long advertising campaign targeting high-interest moments, such as the upcoming Paris Olympics.

Still, Biden's campaign is quietly preparing for a growing chorus of calls for him to drop out of the race in the coming days, making calls to surrogates and calling or texting lawmakers to try to prevent more potential defections.

Biden received backing over the weekend from other key Democrats who had previously raised questions but have now pledged their support, led by House Speaker Nancy Pelosi and Representative James Clyburn of South Carolina.

But especially with the upcoming Democratic Party Convention in mind, the short-term perspective is particularly critical.

Since his disastrous debate performance late last month, donors, strategists, lawmakers and their constituents have urged Democrats to replace him at the top of the ballot before, they argue, it is too late.

Biden's interview with ABC on Friday did not convince some who remain skeptical that he can revive his campaign.

In an appearance on CNN on Sunday, Connecticut Democratic Senator Chris Murphy said Biden “needs to answer voters’ questions.”

However, he added: “If he can do that this week, I think he will be in a very good position and we can get back to what this campaign needs to be.”

Murphy also said he did not know whether a cognitive test would be “impactful” for voters worried about Biden's potential decline.

In his interview on Friday, Biden rejected such tests, arguing that the president's daily rigors were proof enough of his mental acuity.

But California Democratic Rep. Adam Schiff told NBC he would be “happy if both the president and Donald Trump took a cognitive test.”

Trump has said during his own campaign rallies that Biden should undergo such tests, while boasting about his own mental and physical endurance.