Presumptive Republican nominee Donald Trump mocked President Joe Biden after he mistakenly referred to Trump as his vice president during what his administration described as a “big boy” press conference on Thursday.

During a rare solo press conference, Biden made a notable gaffe immediately after he began taking questions from the media, amid growing calls for him to step aside as the Democratic Party’s presidential nominee. Following the mistake, former Trump quickly criticized Biden on Truth Social.

“Crooked Joe begins his ‘Big Boy’ Press Conference with, ‘I wouldn’t have picked Vice President Trump to be vice president, though I think she was not qualified to be president.’ Great job, Joe!” Trump wrote.

Biden’s vice president gaffe was not the only verbal lapse he made on Thursday. Just before the press conference, Biden introduced Ukrainian President Volodymyr Zelensky as “President Putin” at a NATO summit.

In another moment that Trump pointed out on his social media platform, Biden said he was “following the advice of [his] commander-in-chief” before correcting himself to say “chiefs of staff of the military.”

During a rare solo press conference, Biden faced numerous questions from reporters concerning his increasing mental missteps. Despite the scrutiny, he reaffirmed his intention to continue his re-election campaign and serve a second term if victorious in November. At the conclusion of the press conference, Biden was asked to respond to Trump’s criticism of his recent gaffe. Smiling, Biden replied, “Listen to him,” before exiting the room.

A growing number of Democratic lawmakers are urging President Joe Biden to withdraw from the presidential race and allow another candidate to lead the party against Donald Trump. Despite a widely criticized debate performance and his failure to alleviate Democrats’ concerns during an interview with ABC News’ George Stephanopoulos, Biden has consistently stated his intention to remain in the race.