So, what does Joe Biden's decision mean for Republicans? Pete Sessions is a veteran Republican congressman who has represented Texas for nearly 30 years.

"Vice-President Kamala Harris has been an active part of the Biden administration for four years," he tells BBC Radio 4's Today programme.

"I don't think Joe Biden had low polling numbers because of his age - I think it was the policies.

"The question which [we are] asking the United States - am I better off than I was four years ago?"

Sessions adds that it's not certain - in his opinion - that Harris wins the nomination.

Image source, Getty Images

Further to that list of Democrats, the party's congressman in Rhode Island, Gabe Amo, has been speaking to our colleagues on the World Service's Newsday programme.

"I believe that Kamala Harris is ready to go from day one because she has been there," he says.

"She has been the last person in the room with President Biden.

"She has worked hand in glove to respond to some of the crises that our nation has seen, out of the Situation Room, out of the Oval Office and so she is battle-tested and prepared to be our Commander-in-Chief."

Image source, Getty Images

As we've reported, many Democratic Party heavyweights have followed President Joe Biden in supporting Vice-President Kamala Harris:

• California Governor Gavin Newsom - who was tipped as a potential nominee - endorsed Harris last night, writing on X that "no one is better to prosecute the case against Donald Trump's dark vision"
• So did Transport Secretary Pete Buttigieg and Pennsylvania Governor Josh Shapiro, who were also seen as potential candidates
• Former Democratic presidential nominee Hillary Clinton, former President Bill Clinton and former Secretary of State John Kerry also backed Harris

• Alexandria Ocasio-Cortez said she was pledging her "full support to ensure her victory in November"
• Massachusetts Senator Elizabeth Warren, and Minnesota Senator Amy Klobuchar, who ran against Biden for the nomination in 2020, have also both endorsed Harris

Image source, EPA

Gil Duran, who was briefly Kamala Harris's communications director when she was California's attorney general, has spoken to our colleagues on Radio 4's Today programme.

Describing her strengths, Duran says Harris has had a "meteoric rise in politics" and is a role model for women and people of colour in politics.

But he says Harris's journey has not been straightforward.

"She ran a pretty terrible campaign [to be the nominee] in 2020 and dropped out before the primaries. She has had some issues in keeping staff and articulating her values in interviews.

"I think she has the potential to be a great president - she has captured the hopes and aspirations of a lot of Americans. Right now we are going to find out what she is made of."

Image source, Reuters

Image source, Reuters

Since Joe Biden made his announcement 12 hours ago, the Republican nominee Donald Trump has published a flurry of posts on his Truth Social platform.

They include, from three hours ago: "Who is running our Country right now? It’s not Crooked Joe, he has no idea where he is. If he can’t run for office, he can’t run our Country!!!"

In two more bursts, Trump wrote: "Biden never had Covid. He is a threat to Democracy!"

Which came after: "It’s not over! Tomorrow Crooked Joe Biden’s going to wake up and forget that he dropped out of the race today!"

Earlier, Trump also called for the Republican Party to be "reimbursed for fraud" - and said the next TV debate should be held on Fox News, rather than ABC.

Image source, Reuters

Our colleagues at BBC 5 Live have just spoken to Jacob Rubashkin, an editor with non-partisan newsletter Inside Elections, who believes Joe Biden's position was "politically untenable".

"This really does reset the presidential election and pushes us into uncharted waters," he says.

"That debate last month on 27 June was the catalyst. It [Biden's withdrawal] had to happen now if it was going to happen."

Madeline Halpert
Reporting from New York

While Democrats coalesce around Kamala Harris, a lot needs to happen before she is formally chosen as the party's candidate for the November election.

Biden had already won 3,896 delegates - far more than he needed to secure his party's nomination. But now that he's out, they are no longer pledged to him as the candidate.

It will be up to the delegates to choose their new nominee. They are scheduled to meet in Chicago for the party's convention on 19 August.

If the Democratic Party does not come together to support a new candidate, that could mean an open convention for the first time since 1968.

To secure the party nomination, a candidate needs 1,976 delegate votes.

• Read more on the process here

Sam Hancock
Live page editor

Here in London, we on the overnight team are handing over to our daytime colleagues soon.

It's less than 12 hours since Joe Biden withdrew from the US election race, and then endorsed his Vice-President Kamala Harris to replace him as the Democratic nominee for president.

Over the last few hours, a growing number of Democrats - including people like Gavin Newsom and Pete Buttigieg, who had themselves been touted as nominees - have rallied around Harris.

Nothing's official yet, though, so all eyes remain on the Democratic Party to see what comes next.

Updates and reaction are still coming thick and fast - so stay tuned as the team take you through the next few hours.

Thousands of black women across America joined a Zoom call on Sunday night to throw their support behind Kamala Harris.

Longtime Democratic operatives, and senior Harris staffers were also on the call - according to reports - which was hosted by the group Win With Black Women.

Harris’s deputy chief of staff spoke, telling the group that Harris would go after Trump’s age and criminal convictions if she secured the nomination.

“Her role as a prosecutor makes her the ultimate contrast to Trump, the convicted felon. She can make the issue of age and fitness a liability for Trump,” Erin Wilson said, according to the New York Times.

Kamala Harris has not been confirmed as the Democrats' presidential nominee. But the party is falling in behind her - and as our special correspondent Katty Kay reported earlier, thoughts are now turning to who she may pick as a running mate.

As a reminder, the running mate would become vice-president, should Harris win November's election.

Speaking to CNN, Congressman Jim Clyburn of South Carolina suggested three governors could be in the running - Andy Beshear of Kentucky, Roy Cooper of North Carolina and Josh Shapiro of Pennsylvania.

Beshear and Cooper are two Democrats who have both held their own in states that voted for Trump in 2016 and 2020.

And Josh Shapiro - who has a legal background similar to Harris - has been framed as a rising star in the party, due to his long history of winning elections in Pennsylvania, which Democrats see as a must win state in November.

• Read more here

Image source, Reuters

Image source, Reuters

In the hours that have followed Joe Biden's withdrawal from the US election race, a number of celebrities have publicly expressed their support for Vice-President Kamala Harris.

• Actress Jamie Lee Curtis described Harris as "trusted and tested" - adding that she's a "fierce advocate for women's rights and people of colour and her message is one of hope and unity for America at her time of great national divide"
• Singer Katy Perry posted a video message on Instagram, where she sings "it's a woman's world, and you're lucky to be living in it"
• Reacting to President Biden's decision to drop out, talk show host and comedian Jon Stewart published one-word post on X. "Legend"
• And Oscar-winning actress and singer Barbra Streisand wrote on X: "Joe Biden will go down in history as a man who accomplished significant achievements in his four-year term. "We should be grateful for his upholding of our democracy"

The BBC has spoken to Democrat voter Isaiah, who says he's happy to see change at the top of the Democratic ticket - but thinks it may have come too late to beat Donald Trump.

"This is a decision that should have been made a year ago if not longer. That was pretty much my hesitation with [pushing Biden out], in that it just feels a little bit too late in the process.

"Logistically I don’t think we can handle a contested primary. Even though [Harris] is not my number-one-preferred-candidate I think she's fine.

"I'd be surprised if any candidate won against Trump, whether it be Biden, Harris or "insert Democrat here". But I think she gives us a slightly better chance - I think it’s a good decision.

"I'm glad he stepped down but again it just feels like poor planning on the Democratic Party's part. They're always running around like a chicken with their head cut off, it doesn’t scream party cohesion, but I think it's the best path forward."

We've seen Democrats falling in line behind Kamala Harris - but what about Republicans?

JD Vance, Donald Trump's running mate in the US presidential election, earlier described Biden as having been "the worst president" in his lifetime.

Harris "has been right there with him every step of the way," Vance added in a post on X, accusing the current vice-president of lying about Biden's "mental capacity" and "saddling the nation with a president who can't do the job".

Meanwhile, the Republican National Committee (RNC) released a statement listing - what it called - "every single failure we've seen from Joe Biden". It mentioned "the Afghanistan withdrawal, a border crisis, crushing inflation, and America weakened abroad" as examples, saying these had all been "delivered hand-in-hand with Kamala Harris".

"As Democrats fall apart, President Trump is uniting the American people behind his winning agenda," the statement said.

Jamie Whitehead
Live reporter

Image source, Getty Images

If Kamala Harris wins the Democrat nomination for president and then goes on to win the election in November, she will become the first ever woman to win the presidency of the United States.

But it won't be the first time she's held presidential powers.

Back in 2021, when Joe Biden was placed under anaesthesia for a routine colonoscopy, Harris held power for 85 minutes.

During her just-over-an-hour in power, Harris carried out her duties from her office in the West Wing of the White House.

At the time, White House Press Secretary Jen Psaki said a transfer of powers in such circumstances was not unprecedented, and it was part of the process set out in the US constitution.

The 25th amendment, external of the constitution says: "In case of the removal of the president from office or of his death or resignation, the vice-president shall become president."

A temporary transition of power is detailed in section three of the amendment, a constitutional change brought on by the assassination of President Kennedy in 1963.

The official social media account for the Biden-Harris campaign on X, @BidenHQ, has now officially been rebranded to @KamalaHQ.

It is yet another sign of how quickly the apparatus around President Biden’s campaign is being repurposed to support Harris.

In the first post since changing, external from @BidenHQ, the account re-introduces itself to its 455,000 followers by saying : "Welcome to Kamala HQ."

The second post is a Venn diagram meme.

It is, of course, impossible to speculate what posts may come from the account, but earlier this year the Joe Biden campaign used Kendrick Lamar's song "Euphoria" to diss Donald Trump.

Image source, X/KamalaHQ

Sam Hancock
Live page editor

Well, there's been quite the flurry of news lines over the last few hours, so thanks for sticking with us.

Reaction to Joe Biden's announcement on Sunday, that he'd be dropping out of the US election race and no longer seek a second term as president, is still coming in. (The drama that led up to that moment is explored brilliantly in this article.)

As are the names of senior Democrats who are rallying around Kamala Harris and pitting her as the right person to take on Republican nominee Donald Trump. They so far include the likes of Gavin Newsom and Pete Buttigieg, who were themselves widely tipped as potentials to replace Biden.

Nothing's been confirmed by the Democratic Party yet, though.

Trump's yet to mention Harris in the slew of social media posts he's been putting out since the news came - though he's had plenty to say about the man he calls "Crooked Joe Biden".

Despite calls from some Republicans, including House Speaker Mike Johnson, for Biden to resign as president now, the president has insisted he'll stay in office until his successor is chosen. As a reminder, the election is in November but a new president won't be sworn in until January 2025.

Back in the here and now, we're going to continue bringing you all the latest updates and analysis - stick with us.

The BBC has also spoken to Jim, a life-long Republican who was struggling to support Trump again after the former president's criminal conviction.

How will Biden's decision influence his vote?

"When I first heard [Biden was stepping down] I thought this is long overdue.

"But the fact that Biden won’t step down as president altogether means this decision isn’t looking out for the interests of the country, this is looking out for who can win the election.

"He should have passed the baton to whoever and let the party pick.

"I will never vote for Kamala Harris, I don’t think she’s qualified. There’s no way, I don’t think she’s capable.

"I feel like Trump is trending really strongly and I think he’s going to win the election with a very strong margin, electorally and probably even popular vote too. The assassination attempt changed things a lot.

"I’m gravitating toward Trump now. I really have a hard time saying I’d vote for a Democrat."

Ana Faguy
Reporting from Washington DC

The BBC has spoken to several voters to ask what they made of Joe Biden's historic decision to pull out of the election race, as well as his decision to endorse Kamala Harris.

Anna - a Democratic voter from Iowa - says she was going to vote Democratic whoever the nominee was, but she's excited to back Kamala Harris.

"I was hoping Joe Biden would do this and was kind of expecting it today. I think he did what was right for the country and it was a selfless act.

"I wanted to see a younger candidate all along. I think Joe Biden has been a wonderful candidate, but he’s been declining and asking him to serve another four years was asking a lot.

"Who [Harris] picks as her vice-presidential candidate needs to get people excited about this campaign.

"I would love to see two women on the ticket but that would be hard, just because that's the way it is. Realistically there needs to be a man from a swing state.

"Anyone who is saying this isn’t a democratic process is wrong. People who voted for Joe Biden voted for Kamala Harris; picking someone else at this point would be less democratic."

Katty Kay
US special correspondent

As more and more Democratic leaders follow President Joe Biden and endorse Vice-President Kamala Harris, it will now be increasingly difficult for another Democrat to mount a serious, substantive challenge to her and claim that they should be the party’s candidate instead of Harris.

After Biden’s disastrous debate performance a few weeks ago, there was a lot of talk about whether there could be an “open convention” or a “mini-primary” where various prominent Democrats all threw their hats into the ring and made their case. People like Josh Shapiro, Gavin Newsom and Gretchen Whitmer were all discussed as possible candidates.

If Biden had dropped out a month ago, the odds of such a contest would’ve been much more likely.

But now that Vice-President Harris has received the backing of so much of her party’s leadership, the question I hear many Democrats asking tonight is not “Who will the nominee be?” but rather “Who might she pick to be her own vice-president?”

Trump has been riffing on his social media platform Truth Social about Biden’s decision to step aside as the Democratic party’s nominee

“So, we are forced to spend time and money on fighting Crooked Joe Biden, he polls badly after having a terrible debate, and quits the race. Now we have to start all over again,” he writes in a recent post.

“It’s not over!”, he adds, saying it was possible Biden would “wake up [tomorrow] and forget that he dropped out of the race”, in a thinly veiled jab at the president’s age. Biden is 81 years old.

Trump, 78, is yet to mention Kamala Harris - who has been widely endorsed by Democrats as Biden's replacement - in any of his posts.

Image source, Reuters