Biden faces huge test of his mental and physical fitness as he welcomes world leaders to the NATO summit

President Joe Biden's ability to lead and physical and mental fitness will be tested this week as he welcomes world leaders to the NATO summit in Washington D.C.

Former NATO Deputy Secretary-General Rose Gottemoeller told CNBC the stakes are high for the embattled 81-year-old facing calls to drop out of the presidential race.

She said he needs to prove himself to world leaders, some of whom are reportedly concerned about his cognitive decline.

Comedian Jon Stewart took to his Daily Show platform on Monday night to slam the Democratic Party's downplaying of Biden's abysmal showing in his showdown debate with Donald Trump in June.

'For a campaign based on honesty and decency, the spin about the debate appears to be blatant bulls**t, and the redemption tour hasn't gone that much better,' Stewart began.

'We're told that the threat of Trump is so great and the stakes are so high that even bringing up these absolutely legitimate concerns about the president's ability to do the most vigorous job in the world for the next four years is enabling fascism.'

Stewart said that the concerns about another Trump administration 'aren't the only threats our democracy faces,' going on to say that another threat is Biden's campaign being unable to deal with the concerns of voters about the president.

The host described an 'arthritic status quo' that 'erodes confidence and faith in the system of government borders.'

The White House on Monday said that President Joe Biden was not being treated for Parkinson's disease during a bad-tempered press briefing when officials were accused of hiding his medical details.

Press Secretary Karine Jean-Pierre at times struggled to be heard amid a cacophony of questions about the 81-year-old president's health.

But that simply triggered a hubbub from the usually docile front row of reporters who peppered her with demands for a proper explanation.

'Ed, a little respect please,' said Jean-Pierre to CBS News senior White House correspondent Ed O'Keefe, who wanted to know why she was not prepared to discuss details of Dr. Kevin Cannard's visits, when they were already a matter of public record, revealed by visitor logs.

Biden's personal doctor, Kevin O'Connor, broke protocol Monday, releasing a letter explaining why Dr. Kevin Cannard visited the White House eight times in eight months.

The letter came after a frenzied day of speculation over Biden's ailing health, which has been in question since his disastrous first debate with Donald Trump, with Press Secretary Karine Jean-Pierre adding fuel Monday by saying the president has seen a neurologist three times.

Cannard is a movement disorder specialist at the Walter Reed Medical Center in Maryland.

On Monday, O'Connor released a letter with the permission of both Biden and Cannard and confirmed that Cannard had been the neurologist who evaluated the president at each of his three physicals since he took office.

After Biden's disastrous debate performance against former President Donald Trump on June 27,  questions have swirled over the 81-year-old's physical and mental health - and whether he should withdraw from the race.

So far, only a small cohort of Democrat lawmakers have called on the president to withdraw from the race while many more have thrown their support behind Biden.

The Squad leader confirmed Monday she is on board with the president's leadership.

'I have spoken with [Biden] extensively,' AOC revealed. 'The matter is closed. Joe Biden is our nominee.'