President Joe Biden was defiant during a rare news conference ... fighting back at critics who are calling for him to drop out of the race.

POTUS is trying to show voters he's still got the mental chops to beat Donald Trump and carry out the duties of president for another term ... and while he made a few trademark flubs -- like saying "Vice President Trump" in referring to Kamala Harris -- he was confident in his ability to win another election, and pushed back on narratives about his mental fitness.

Biden called his debate disaster a "stupid mistake" ... saying he was off his game because he traveled all over creation in the weeks before ... including from Italy to L.A. to go to the fundraiser with George Clooney. He said he won't make that mistake again.

The Prez also poo-poo'd stories about him being no good after 8 PM ... Biden says the narrative out there is false and he just wants fundraising events moved up in the day so his supporters can get home earlier. He says he's trying to pace himself more and his schedule remains jam-packed after the debate.

Biden took some shots at Trump too ... saying the former president just rides around on his golf cart these days while he gets important policy passed.

There's a lot of pressure on Biden here, both internally and externally, with White House press secretary Karine Jean-Pierre even hyping this appearance up as a "big boy" press conference.

Biden's so far been defiant amid growing calls for him to drop out of the race, saying he has no plans to change course ... and he seemed pretty peeved with all the noise, and it came across in his remarks.

TMZ broke the story ... 'GMA' anchor George Stephanopoulos is on video telling a passerby he doesn't believe Biden could serve another term. George has talked to Biden more than most recently, including a recent one-on-one interview.

A day before Biden's big press conference, George Clooney published an op-ed in The New York Times calling on Biden to make way for a different candidate to challenge Trump.

Clooney said his interactions with Biden at last month's Los Angeles fundraiser left him feeling like Biden "was not the Joe 'big F-ing deal' Biden of 2010. He wasn't even the Joe Biden of 2020. He was the same man we all witnessed at the debate."

Michael Douglas also went on TV and seconded Clooney's opinion about Biden ... saying on "The View" that George was making a "valid point."

Biden has some folks in his corner ... including D.L. Hughley and Joy Behar -- and California Governor Gavin Newsom continues to go to bat for POTUS.

Those supporters are probably feeling better now ... and Biden's making it clear he's not going to give way to another candidate.