Seeking to revive his struggling reelection campaign, President Joe Biden held a rare rally in Detroit on Friday, telling a cheering crowd he wasn't going to leave the race and warning that Republican Donald Trump poses a serious threat.

Biden, 81, is trying to shift the conversation from his mental sharpness and a growing number of Democratic defections to the impact of another Trump presidency, as he tries to reboot his campaign after a shaky debate performance on June 27.

"I am running and we're going to win," he said to a crowd that carried "Motown is Joetown" signs and chanted: "Don't you quit."

"I'm the nominee," he said. "I'm not going anywhere."

Pointing to the press area in the gym, Biden said: "They've been hammering me," to which the crowd booed. "Guess what, Donald Trump has gotten a free pass," he added.

"Hopefully with age comes a little wisdom," Biden said, in a defiant and sometimes gleeful performance. "Here's what I know - I know how to tell the truth, I know right from wrong ... and I know Americans want a president, not a dictator."

Biden also laid out what he intended to do with his first 100 days of a second term, including codifying abortion rights, signing the John Lewis Voting Rights Act, ending medical debt, raising the minimum wage and banning assault weapons.

These sweeping changes would be difficult or impossible without Democratic majorities in both houses of Congress.

While union and religious leaders attended, Michigan's governor Gretchen Whitmer and its Democratic senators, Debbie Stabenow and Gary Peters, did not.

Earlier on Friday, Biden made a surprise stop at a restaurant in a Detroit suburb, where he told diners he planned to "finish the job," and said: "I promise you... I'm okay."

Biden got a boost on Friday when two prominent Democrats - Representative James Clyburn and California Governor Gavin Newsom - said he should stay in the race.

On Friday afternoon, United Auto Workers president Shawn Fain, who had previously said he was worried about the president's chances, praised Biden for standing "with the working class," without using his name.

But there were signs that his support was weakening elsewhere, as two more lawmakers called on him to drop out.

"It is time to move forward. With a new leader," Representative Mike Levin, from California, said in a statement. Levin, like many others who have called on Biden to leave the race, faces a competitive reelection battle of his own this year.

Since the debate, at least 19 lawmakers have urged Biden to step aside so the party can pick another candidate, as have some donors, Hollywood stars, activist groups and news outlets.

Biden retains support from key figures in the party, however, less than five months from the Nov. 5 election.