Joe Biden's withdrawal from the US election offers Democrats the precious opportunity for a late reset in a campaign that was going badly -- but ushers in an uncertain few weeks that will make or break their chances against Donald Trump. Grassroots voters will hope to be better served than they were during the initial nomination race, when Democrats put up no serious competition to the 81-year-old president's candidacy despite worries over his age.

"Remarkable leadership shown by Joe Biden. Now it falls to the (Democrats) to show equal leadership by having an open process to determine the best candidate(s) to take on Trump-Vance in November," said entrepreneur Andrew Yang, who ran against Biden for the party's nomination in 2020.

"The goal should be simple -- to win," he wrote on X.

With Biden's endorsement, Vice President Kamala Harris, 59, starts out in the strongest position to replace him and may be able to consolidate support in the coming days -- a coronation that would allow Democrats to turn their fire on Trump.

If that doesn't happen then a chaotic -- and potentially damaging -- fight for the votes of almost 4,000 delegates at the Democratic National Convention in Chicago in just four weeks looks more likely. The party's national chairman Jaime Harrison promised a "transparent and orderly process" to replace Biden, the first president since Democrat Lyndon B. Johnson in 1968 not to seek a second term.

His campaign began collapsing after his dismal debate performance against Trump last month prompted concerns over his advanced years and mental capacity.

Trump, who would be 82 at the end of a second term, appeared more robust -- but opposition from a younger candidate would turn the spotlight on his speeches and interviews, where he often appears incoherent and confuses names.

Several rising Democratic stars -- all younger than 60 -- have been floated as Biden alternatives, including governors Josh Shapiro, Gretchen Whitmer and Gavin Newsom. Whitmer immediately appeared to suggest she would not be running and Newsom has previously said he would not challenge Harris, but neither explicitly ruled themselves out.

A campaign led by Harris and backed up by a moderate midwestern running mate could present the best chance for Democrats -- helping turn out more women, who historically vote in greater numbers than men and are a weakness for Trump. Harris would also give Democrats a chance to redefine the race at their convention as a clash of cultures between a former prosecutor and a convicted felon.

"While she has largely been in the shadows for the last four years it is time for Kamala the prosecutor... to make her comeback," said Sara Sadhwani, an assistant professor of politics at Pomona College in Claremont, California.

A new survey from Democratic polling firm Public Policy Polling released Thursday found that Harris -- with the right running mate -- could defeat Trump and Vance in Pennsylvania and Michigan, two of the three "Blue Wall" states seen as critical to electing a Democratic president.

Ryan Waite, a political marketing expert from Brigham Young University-Idaho, told AFP a contested nomination might actually benefit Democrats by depriving Trump of headlines for several months.

"A new Democrat candidate will benefit from a short vetting window and significant earned media impact right before the election," he told AFP. "The story is now who will it be and that will take the air out of the room through their convention."

Any new nominee other than Harris would represent a clean break and be immune from much of the criticism directed at the Biden administration over high inflation, the border crisis and the chaotic withdrawal from Afghanistan. But while Harris would likely inherit all of the Biden campaign's cash reserves -- about $94 million as of July -- other candidates may have to share the money with down-ballot candidates.

They would also have to make the case anew for endorsements from some of the Biden-Harris campaign's most influential backers, such as the United Auto Workers union. Harris herself could also face this problem, of course.

Republicans have suggested that they might challenge any late substitutions in the courts but Marc Elias, the Democrats' top election lawyer vowed that the eventual nominee would be on all 50 state ballots. "There is no basis for any legal challenge. Period," he said. (AFP)