Joe Biden’s catastrophic showing at the debate with Donald Trump last month was the worst kind of milestone for the US president – it marked the beginning of the end of his bid for re-election. Trump was formally nominated for the Republicans at the party’s convention, held in Milwaukee, last week, and now Biden will be out of the race.

So who will be at the top of the Democratic ticket against Trump now?

Biden won the Democratic primaries earlier this year but would not have officially become the party’s candidate for president until endorsed at the 2024 Democratic national convention in Chicago, which takes place from 19-22 August. And it has all fallen apart a month before that.

There was no formal mechanism to replace him as presumptive nominee, and such a move would have been the first time a US political party had attempted to do so in modern times. With Biden ultimately quitting of his own volition, he will hope to make the process to choose his replacement smoother. But it will not be straightforward and the immediate road ahead is not clear.

On Sunday afternoon, Biden followed his announcement – that he was staying on at the White House as president but stepping aside from his re-election campaign – by saying he now endorses his vice-president, Kamala Harris, to become the Democratic nominee for president.

She is the first female US vice-president and would become the first female president of the United States if she is officially nominated by the party at its convention and beats Trump in November. Not long after Biden, Bill and Hillary Clinton also endorsed Harris for the top spot, issuing a “statement from President Clinton and Secretary Clinton”.

Hillary Clinton almost became the first woman to be US president, seeking to shatter what she called “the hardest, highest glass ceiling” and was the first female nominee from either of the two major parties in history. But she failed to win it for the Democrats and for women in 2016 as Trump took a shock victory for the Republicans.

Biden’s endorsement of Harris will carry weight with delegates at the convention but is not binding.

The Chicago convention was supposed to be a highly choreographed event, a formality, for the Biden-Harris ticket. Now things are up in the air.

The Democratic National Committee’s chair, Jaime Harrison, said in a statement that the party would “undertake a transparent and orderly process” to select “a candidate who can defeat Donald Trump in November”.

So who are the most likely candidates to head the Democratic bid for the White House in November?

Kamala Harris

Biden’s vice-president was always the most logical pick, as someone already deputizing for the commander-in-chief. But now Harris is the obvious frontrunner for the nomination. However, she has been widely criticised for not carving out more of her own role in the Biden administration and has poor polling approval ratings, suggesting she would struggle against Donald Trump in the glare of an election campaign. The 59-year-old was backing Biden after the debate. Then support for him drained away, and now the spotlight is being trained firmly on Harris.

The most senior party leadership, in the shape of the Democratic House speaker emerita, Nancy Pelosi, Chuck Schumer, the Senate majority leader, and Hakeem Jeffries, the House minority leader, reportedly privately agreed with dozens of members of Congress who have been publicly calling on Biden to quit the race. That number continued to grow over the weekend, until Biden did as they were urging. But few until Biden on Sunday specified who should replace him.

Harris was chosen as Biden’s running mate when she was a US senator for California, having been attorney general of the state and previously a district attorney based in San Francisco. She ran an unsuccessful campaign for the presidential nomination in 2020 that never made it to the primaries. As vice-president she was given difficult briefs including immigration but has most assuredly found her footing defending reproductive rights, in the wake of the US supreme court overturning Roe v Wade in 2022.

JB Pritzker

The 59-year-old governor of Illinois would be one of the wealthiest of possible picks. He can flourish his credentials of having codified the right to abortion in Illinois and declaring it a “sanctuary state” for women seeking abortions. He has also been strong on gun control, and legalised recreational marijuana.

Gretchen Whitmer

The Michigan governor, 52, was on the shortlist for VP pick for Biden in 2020, and a strong showing in the midterms for the Democratic party was in part attributed to her governership. She has been in favour of stricter gun laws, repealing abortion bans and backing universal preschool.

Josh Shapiro

The Democratic governor of Pennsylvania has been a rising political star in this crucial swing state for years but shot to national prominence in fall 2022 when, as state attorney general, he won the gubernatorial race over an extreme Trumpist Republican. His name has been on lips of late and the reckoning since Biden stepped aside is if Harris wins the nomination, Shapiro would be a strong pick as her running mate. Alison Dagnes, a professor of political science at Shippensburg University in southcentral Pennsylvania, told the USA Today media network that Shapiro generating buzz because he’s shrewd, tech-savvy and a proven winner in the battleground state with the most electoral college votes.

Joe Manchin

Joe Manchin, the center-right West Virginia senator, has been considering running for the Democratic nomination for years. He has served in the Senate since 2010 and is chair of the Senate energy and natural resources committee. He has worked hard to shore up his support in West Virginia, while often being outspokenly critical of the Democratic party. Facing a tough re-election race, Manchin announced last year he would not seek another term in the US Senate, and he left the Democratic party in May to register as an independent. On Sunday, reports emerged he was considering re-registering as a Democrat and challenging Harris.