This risky, unprecedented move could be the circuit breaker the Democrats need

Yet, it is still unprecedented in the modern era for a sitting president to drop out of a presidential race this late in the process. We are really in uncharted waters for American politics.

So, where does the race go from here - and what does it mean for both the Democrats and Donald Trump, the Republican nominee?

Biden has endorsed his vice president, Kamala Harris, as the nominee, which I believe he was always going to do. Not anointing Harris as his successor would have been politically unthinkable for Biden simply because it would have undermined his own decision-making when he chose her to be his running mate before the 2020 election.

His endorsement will also have a big impact on the decision-making of the Democratic Party as a whole. Anybody who challenges Harris now might well be taking on the establishment of the party. And the party would be wary of increasing perceptions of disunity at this point.

So, it certainly seems the Democrats will unite behind her as the candidate.

That said, it's not out of the realm of possibility that she could be challenged by another presidential hopeful. This may well depend on her performance - and the broader reactions to her - in the next couple of days.

Potential rivals will have political ambitions of their own - we can't discount that.

There will also be significant calculations about whether Harris can do the job - if she can win a national election campaign against Trump. Unfortunately, there's a lot of wariness in the United States about what it would mean to be have a Black woman running against Trump, and how that would be received.

And lastly, there is a lot of very real angst within the Democratic Party - and more broadly - about what a Trump victory would mean for the country. In many people's views, the best way to avert what Democrats see as a potential catastrophe is to nominate the candidate with the best chance of beating him. Whether or not the party settles on Harris as that person remains to be seen.

The risks are quite obvious. There's a real wariness about the unprecedented nature of this decision and how it will rattle the party.

One risk is Harris turns out not to be the right candidate and that the presidential race would be difficult for her to win, especially given how badly her first campaign for the presidency went in 2020.

The risks to American politics more broadly are also quite serious, given the level of vitriol and hatred that will be directed at the Democratic Party if a Black woman is the head of the ticket.

But I think it's also entirely possible this is the circuit breaker the Democrats needed. The party had been slipping into a morass of despair after Biden's debate performance against Trump last month and the subsequent assassination attempt against the former president. In a recent poll, two-thirds of Democrats said they believed Biden should withdraw from the race.

Given that both Biden and Trump are so disliked by the American population, Harris (and her running mate) could represent a huge opportunity for the Democrats.

A key strength of Harris is on the critical issue of reproductive rights. She has owned this issue in the campaign so far because Biden has difficulty with abortion politically as a devout Catholic - and we saw how he struggled with the messaging on this issue in the debate.

It's entirely possible it will be a hugely mobilising factor in this year's election, as it was in the 2022 midterm elections. As such, Harris would have a significant advantage in persuading the Democratic base to turn out to vote on that issue, in addition to other voters.

Assuming Harris does become the nominee, Trump's pick of J.D. Vance as his vice presidential running mate may turn out to have been a mistake. Like much of the movement that supports Trump, Vance is virulently anti-abortion and committed to further regulations on reproductive rights.

This could be a significant vulnerability for Trump, who doesn't perform as well with women voters compared to men.

And it seems the Trump campaign knows this, which is why it has been campaigning against Harris even before it was clear Biden was going to drop out.

The attacks on Harris from the right have been very brazen in their misogyny and racism. For example, the New York Post published a column earlier this month saying Harris would be the country's first "DEI president", referring to diversity, equity and inclusion. This is a way of dismissing anybody who's not white and not a man as not being worthy of the position they've earned.

Trump rose to national prominence prosecuting the same kind of racist campaign against the presidency of Barack Obama.

I think we can expect there to be a push for the Trump campaign not to go down that road any further. But we've seen in the past how Trump completely ignores that kind of conventional wisdom, and just how much that incites his base.

This remains a dangerous and unpredictable era in American politics. There are no rules for what happens next.