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.