Who will replace Biden and why has he quit?

Top Democrats have thrown their support behind Kamala Harris to be the party's US presidential nominee after Joe Biden withdrew from the race.

Biden, 81, announced on Sunday he would step aside in the "best interest of my party and the country" amid mounting concerns from those within his party in recent weeks that he would be unable to defeat Republican nominee Donald Trump in November's election.

Biden endorsed his vice-president Harris as the Democratic nominee, saying: "Democrats - it's time to come together and beat Trump."

Harris also has the backing of senior Democrats such as California governor Gavin Newsom and transportation secretary Pete Buttigieg, who had both been viewed as potential nominees. Former president Bill Clinton, and former presidential nominees Hillary Clinton and John Kerry, have also voiced their support for Harris.

All 50 state Democratic party chairs have backed Harris, although she has not yet received the support of former president Barack Obama and ex-House of Representatives speaker Nancy Pelosi - Obama had reportedly expressed concerns about Biden's chances.

Why has Joe Biden left the race?

In a letter posted on X, formerly Twitter, Biden said it had been the greatest honour of his life to serve as president.

Biden's announcement comes after weeks of speculation following his disastrous TV debate performance against Trump in June, and an ongoing series of gaffes that led to questions over his capability of standing for another term.

The president has been resting at his beach home in Delaware after it emerged that he had contracted COVID, but he has not publicly suggested that his health is behind his decision to withdraw from the race. In his announcement he said he will "speak to the nation later this week in more detail about my decision".

• Read more: The 48 hours that consigned Joe Biden’s 2024 candidacy to history(The Guardian)

Who will replace Joe Biden?

Biden may have stepped down as the Democratic candidate, but he has not stepped down as president, saying he plans to "focus solely on fulfilling my duties as president for the remainder of my term".

The 46th US president was not yet officially the Democrats' candidate for 2024, although he had already secured the number of delegates needed to officially gain his party's nomination at the Democratic National Convention on 19 August.

However, now Biden has withdrawn, the party is left having to find another nominee in what is an unprecedented situation with no clear guidelines for what happens next.

There is the possibility that the party could hold a series of mini-primaries ahed of the convention to decide upon a candidate; they could then agree the candidate in a virtual role call ahead of the convention (which is what was previously planned to firm up Biden's nomination).

However, the event may become an 'open' convention when no candidate has a clear majority and those in the running are left trying to persuade delegates to vote for them; or it could be a 'brokered' convention, in which the delegates remain unbound but have taken part in off-books deals backing a particular person.

• Read more: Joe Biden reached pinnacle of US politics but became liability for Democrats(Sky News)

Is Kamala Harris automatically the new Democratic candidate?

If Biden had stepped down as president, vice-president Kamala Harris would automatically replace him - but she does not automatically become the Democrat's candidate for the 2024 election.

Biden has endorsed Harris, saying: "My very first decision as the party nominee in 2020 was to pick Kamala Harris as my vice-president. And it’s been the best decision I’ve made.

"Today I want to offer my full support and endorsement for Kamala to be the nominee of our party this year. Democrats — it’s time to come together and beat Trump. Let’s do this."

But that doesn't mean Harris is automatically the new candidate, though many think she is the obvious choice for the job.

Delegates are now free to vote as they choose, which means even though Biden has endorsed Harris, delegates could decide someone else is better placed to be the party's presidential candidate.

• Read more: Who could challenge Kamala Harris in race to replace Joe Biden?(Sky News)

What is Kamala Harris' background?

Born in California in 1964, Kamala Harris studied political science and economics at Howard University before going on to receive a law degree from the University of California.

She has had a long career in law, becoming the first female district attorney for San Francisco in 2004, then becoming attorney general of California.

In 2016, she won the US Senate race in California and launched a campaign for the Democratic presidential nomination in January 2019, but dropped out of the race in December that year.

She is is the first woman to serve as vice-president as well as the first Black woman and person of South Asian descent to serve in the office.

• Read more: This is Kamala Harris's moment - everything you need to know(Los Angeles Times)

What else do I need to know about Kamala Harris?

Harris has stayed loyal to Biden, and referred to him as a "leader who fights for the American people".

Asked in 2023 if she could step in for Biden if he became unable to continue as president, she said: "Yes, I am, if necessary," but added: "But Joe Biden is going to be fine."

With Harris now in the running to be the Democrats' presidential candidate, her stance on key issues is more under scrutiny than ever, from healthcare to immigration and everything in between.

While the current vice-president's views on some areas are in line with Biden's, there are some policies where she could differ, such as abortion and climate change, where it is believed she would push harder than the president.

• Read more: Celebrities endorse Kamala Harris for president in droves as Joe Biden drops out(PA Media)

What about Kamala Harris's odds in the polls against Trump?

As speculation swirled around Biden's suitability as the Democratic candidate to take on Trump following his poor debate performance, polls started to look at how Harris could stand up against the former US president.

Many put Harris on a similar level as Biden, though some gave her a slight edge.

In one CBS News/YouGov poll Trump led Harris by three points (51% to 48%), compared to a five-point difference to Biden (52%-47%), while an Economist/YouGov poll found Biden would lose to Trump by two points (41% to 43%), and Harris would perform slightly worse, losing to Trump 39% to 44%.

On Monday, following Biden's announcement, Oddschecker put Donald Trump at 1/2 to win the US presidential election, with Harris at 2/1.

• Read more: Who could be Kamala Harris’s running mate now Biden has dropped out the race?(The Independent)

Could there be a challenge and a different candidate?

Other than Kamala Harris, other names who could replace Biden as the Democrats' candidate include Michigan governor Gretchen Whitmer, who famously sparred with Donald Trump over the federal government’s response to the COVID pandemic, prompting Trump’s reference to her as "that woman from Michigan" during a White House briefing.

California governor Gavin Newsom is also seen as one of the Democratic Party’s rising stars and was expected to launch a presidential bid in coming campaigns, but he has backed Harris.

Other names include Kentucky governor Andy Beshear, Maryland governor Wes Moore and senators Amy Klobuchar and Cory Booker.

Michelle Obama's name also crops up, with many thinking she is well placed to beat Donald Trump, but the former first lady has repeatedly said she will not make a foray into politics.

• Read more: Who could be Kamala Harris’s running mate now Biden has dropped out the race?(The Independent)

What happens at the Democratic Convention?

At the Democratic Convention, which takes place next month, delegates from all 50 states attend to officially announce a candidate.

The decision is based on voting in the primaries, which meant it was already known that Biden had secured a majority - he needed the votes of 1,976 delegates to win the nomination and nearly 3,900 had already pledged their support for him.

However, now those delegates are free to vote for whoever their want, which means if a virtual vote ahead of the convention doesn't give a clear idea of who is replacing Biden, the convention itself could be a messy affair with potential candidates vying for support.

• Read more: Kamala Harris quickly moves to secure endorsements heading into Democratic convention(The Independent)