The United States President Joe Biden faces a critical week Monday that will test his standing at home and abroad, as moves to force him to quit his fight for a second White House term gather pace.

The 81-year-old has so far defied calls to step aside after a disastrous debate against election rival Donald Trump last month threw into stark relief fears that he is too old to serve as president until 2029.

Allies have warned he needs to do more to convince his party and the public that, as he insists, only he can beat Trump at the ballot box.

“This week is going to be absolutely critical,” Democratic Senator Chris Murphy told CNN on Sunday.

NATO leaders will also need reassurance as they gather in Washington for a summit this week, with many European countries fearful of 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.

After a barnstorming day of campaign events in swing state Pennsylvania Sunday, Biden has no public events scheduled for Monday, which he is expected to spend preparing for the summit.

First Lady Jill Biden, a fierce defender of the president, is scheduled to campaign for him instead in Georgia, Florida, and North Carolina.

And Democratic lawmakers will return from a brief recess to Capitol Hill under pressure to either fall in line behind the president or urge him to step aside.

Tuesday, when the NATO summit begins, could prove a turning point: Democratic congressmen are expected to hold their regular caucus meeting, which may see the drumbeat of dissent that has dogged Biden since the debate intensifies.

* Clock ticking for Democrats –

In recent days, five Democratic lawmakers have publicly called for him to drop out of the race, while party stalwarts such as former House Speaker Nancy Pelosi have said that questions about his health are “legitimate.”

On Sunday four senior congressmen said on a call with party lawmakers that it was time for Biden to bow out, according to US media.

With election day just four months away, the clock is ticking on any move to replace Biden as the nominee, and Democrats will be scrutinized for any signs of more open rebellion.

As for the public, Biden’s next major test before the increasingly unforgiving glare of the cameras is set to be a press conference on Thursday, during the NATO summit.

On Friday he picks up the campaign trail once more, heading for the battleground state of Michigan, before going to his beachside home in Rehoboth, Delaware.

Biden and his team appear determined to dig in for the siege, with the campaign unveiling an intense schedule for later in July, including an avalanche of TV spots and trips to key states.

But Democrats who have lost faith warn that the threat of a second Trump presidency — one in which, among other things, he has threatened to target “the enemy within” — makes the stakes too high to take any chances.

“There’s only one reason” the race between Trump and Biden is close, Democratic Representative Adam Schiff told NBC on Sunday.

“And that’s the president’s age.”