After weeks of pressure, conjecture, and handwringing at the highest levels of the Democratic Party, President Joe Biden has decided to step aside from the 2024 presidential race.

The president issued a written statement Sunday afternoon, saying he believes it is in the country’s best interest for him to step down, adding that he will address the nation later in the week about his decision. Biden’s decision comes after weeks of public and private entreaties from Democrats who have concluded that he is unlikely to defeat former President Donald Trump.

Moments later, Biden released another statement endorsing Vice President Kamala Harris to take his spot atop the ticket. Some other leading Democrats — Bill and Hillary Clinton, the Congressional Black Caucus, Sens. Tim Kaine (D-VA) and Tammy Baldwin (D-WI) — quickly endorsed Harris, too, revealing a desire in the party to quickly unite behind her.

Such endorsements do not make Harris the presidential nominee — the new nominee will be officially chosen by the thousands of delegates attending the Democratic National Convention beginning August 19. But if such endorsements keep piling up, they may deter any other prominent Democrats from challenging Harris and make her nomination all-but-assured.

Alternatively, other Democrats could put their names in contention. What would happen after that is unclear. Some in the party have spoken of a “blitz primary” or a truly “open convention” where delegates are freed up to support whomever they like. Others view this as chaotic folly. Whether this is at all likely will first depend on whether anyone else jumps in, though.

What is clear is that the presidential contest is entering uncharted territory for the modern era. Since the rise of the modern nomination process in the 1970s, no winner of the primaries has quit the race before the convention. And no major-party nominee has entered the race so close to November.

It’s worth pausing to appreciate just what a dramatic turn of events this has been. It was just a few short months ago that President Biden sailed through the primary season with barely any opposition. But now, his party support has collapsed under him to the point where he effectively can’t continue his campaign.

The catalyst for this collapse was Biden’s June 27 debate performance, which will surely now go down as the most catastrophic general election debate in US presidential history. But what the debate actually did was supercharge concerns about Biden’s electoral prospects, stamina, health, and capacity that have long simmered below the surface of Democratic politics.

The doubts about Biden did not begin with the debate, but the debate suddenly made them impossible to ignore — and spurred the party to take action.

What will happen next, though, is far from clear. Some Democrats profess to believe that Biden was a uniquely flawed nominee because of his age, and that a different nominee could perform much better against the unpopular Trump. But others are more pessimistic, suspecting that the party faces deeper electoral problems and that any replacement nominee is going to have a very tough road ahead.

At the start of Biden’s first term, many in the political world questioned whether he would run for a second term, for two reasons: One is his age, since he’s already the oldest US president ever and would be 82 by Inauguration Day. The other is his bad polling numbers; his job approval has been dismal since late 2021.

But Biden doesn’t ever appear to have seriously considered not running. He said from the start of his term that he intended to run again, and when Democrats’ midterm election performance was better than many expected, it seemed to bolster his case that he could. So he made his reelection campaign official in April 2023.

All the party’s rising stars opted against launching a risky primary challenge to an incumbent president — doing otherwise would have earned them intense criticism in the party for dividing Democrats and damaging Biden’s chances in the general election. So Biden easily fended off challenges from the likes of Rep. Dean Phillips (D-MN) and author Marianne Williamson.

But doubts remained. Biden skeptics raised eyebrows at his skipping the president’s traditional pre-Super Bowl interview, at his reluctance to do press conferences, and at a special counsel report’s claims that he seemed to have a poor memory. In February, New York Times columnist (and Vox co-founder) Ezra Klein caused a stir by writing that Democrats should choose a different nominee at the convention, because Biden did not seem up for the rigors of the campaign.

Then, Biden gave a reasonably spirited performance at his State of the Union address in March, temporarily putting those concerns to rest. But it did not improve his polls — by then, he’d consistently trailed Trump nationally and in almost every swing state for months. He looked on track for defeat.

That’s reportedly why Biden’s team made the surprising proposal to debate Trump so early, a very unusual move. The hope was that Biden could, with a vigorous performance, reset the race, refocus attention on Trump, and address concerns about his age again.

The opposite happened: Biden’s halting performance put a spotlight on his capacity to campaign (and serve another four years) that never went away.

He lost the media first. In the hours after the debate, various liberal commentators called on him to quit the race. Mainstream journalists, sensing scandal and a cover-up, intensely focused on reporting out Biden’s true condition.

Still, Biden hoped that he could keep the party on board by doing a flurry of interviews and public events, showing the debate was a one-off. Some of these went reasonably well, others less so. But his fundraising plummeted, and the public and private pressure from Democrats urging him to step aside just kept increasing — until Biden finally saw the writing on the wall.

Obviously, much will depend on who the nominee is. Clearly, Vice President Harris is the top contender at the moment. If the party simply falls in line behind her, there may not be much suspense by the time the Democratic convention kicks off on August 19. If there is a serious contest, though, strap in for a roller coaster ride over the next month.

But there’s another big underlying question that we don’t yet know the answer to. Namely, how much are Democrats’ electoral woes about Joe Biden himself — and how much reflect deeper problems for the party as a whole?

Some argue that Democrats are doing fine, and that it’s only Biden that’s been the problem; that voters have soured on the president because of his age or perceived incapacity to serve another four years, and because Trump remains flawed and unpopular, and another Democratic nominee would likely beat him. For instance, Democratic Senate incumbents are polling significantly better than Biden. That implies to some that with a different name at the top of the ticket, the party would be in great shape.

Others argue that Democrats’ problems will likely outlast Biden. Unhappiness with inflation under Biden, as well as unfolding foreign tumult, may have convinced voters that Trump wasn’t so bad after all. Public opinion on key issues has moved right. And there isn’t much polling showing any possible Democratic alternative beating Trump.

Some of these pessimists still believe it was worth pushing out Biden since he had little prospect of turning things around, but they emphasize that any replacement would be the underdog and face a very tough road ahead.

All we know for sure now is that Democrats have decided to roll the dice, ditching their unpopular nominee in hopes of improving their odds in a contest they were on track to lose.

Update, July 21, 3:50 pm: This piece was originally published earlier on July 21 and has been updated to include more information about the Democratic National Convention’s options going forward.