NATO summit becomes Biden's biggest test of his mental stamina: What Joe, 81, needs to do to prove he can lead for four more years on the world stage

President Joe Biden faces a test of his stamina with the start of the NATO summit on Tuesday, where voters in America and leaders from abroad will be watching how he holds up after his car-crash debate with Donald Trump.

The three-day fitness test will begin Tuesday evening when Biden, 81, marks the 75th anniversary of the organization. It will continue through meetings and a dinner with world leaders on Wednesday and accumulate in a press conference on Thursday. 

Any stumble could increase calls among Democrats for him to exit the presidential race. A strong performance will shore up his argument he is capable of handling the job.

The NATO summit will be a three-day test of Joe Biden's fitness and leadership

The NATO summit will be a three-day test of Joe Biden's fitness and leadership

Kurt Volker, a former US ambassador to NATO, said foreign leaders will arrive in Washington looking for reassurance in Biden personally but also in America's commitment to NATO. 

'Is it going to be President Biden? Is he capable of that? Is he going to run for reelection, is he going to get reelected? If so, what does that look like?' Volker told CNN on Monday.

National Security Council spokesman John Kirby said the president didn't need to reassure allies following the debate. 

'We're not picking up any signs of that from our allies at all,' he said Monday. 'Quite the contrary, the conversations that we're having with them in advance, they are excited about this summit. They are excited about the possibilities.' 

The gathering of the leaders from the 32 NATO countries — plus Pacific partners Australia, Japan, New Zealand and South Korea, as well as Ukraine — is expected to be one of Biden's last appearances at an international forum before Election Day.

It also comes before the Republican National Convention in Milwaukee next week. 

Biden also has taken credit for the expansion of NATO. Both Finland and Sweden have joined in the aftermath of Russia's February 2022 invasion of Ukraine. 

But, in a reminder of his age, Biden is the only head of government who was alive during NATO's founding. 

Biden has made 'restoring America's place in the world' a major policy platform after Donald Trump's years of isolationist policy.

He cited his foreign policy record as a reason he should remain his party's nominee.

'I expanded NATO. I solidified it,' Biden said on MSNBC on Monday. 'I made sure that we're in a position where we have a coalition of people of nations around the world to deal with China, with Russia. With everything that is going on in the world. We're making real progress.'

But leaders need reassurance he is up for the job. 

'I think this NATO summit in Washington is an important moment for Joe Biden, he really has to show that he is with it, he's ready to continue to lead not only the United States of America but the NATO alliance,' former NATO Deputy Secretary-General Rose Gottemoeller told CNBC. 'And so we will see in coming days just how well the U.S. president does.' 

NATO Secretary General Jens Stoltenberg threw out the first pitch at Washington Nationals Park; he was joined by U.S. Secretary of State Anthony Blinken and Julianne Smith, the Permanent Representative of the United States to NATO

NATO Secretary General Jens Stoltenberg threw out the first pitch at Washington Nationals Park; he was joined by U.S. Secretary of State Anthony Blinken and Julianne Smith, the Permanent Representative of the United States to NATO

Members of the media gather at the NATO summit location in Washington DC

Members of the media gather at the NATO summit location in Washington DC

Trump, meanwhile, has suggested that in a second term he would not defend NATO allies who didn't pay their full dues -  2% of their respective GDP. The former president also has questioned the amount of aid to the Ukraine.

During the three-day meeting, Biden is expected to endorse new aid for Ukraine. 

He also will hold bilateral meetings with Ukrainian President Volodymyr Zelensky and new British Prime Minister Keir Starmer.