Joe Biden's spotty debate performance immediately triggered new questions from worried Democrats about whether he would leave the presidential race.
It would not be an easy process since Biden is already the Democrats' presumptive nominee and the overwhelming choice of primary voters. He faced little opposition during the primary season, and the fact that he won nearly all of the party's delegates means it's very unlikely he'd be forced out of the race against his will.
"This isn't the '60s. Voters choose the nominee. He is the nominee," said CNN analyst and Democratic strategist David Axelrod, reacting to Biden's performance at the debate Thursday night on CNN.
That current primary system, which empowers primary voters over party bigwigs, essentially sprang from discontent after Democrats selected Vice President Hubert Humphrey as their nominee in 1968. Even after President Lyndon Johnson bowed out of the presidential race that year, recognising his fading popularity and opposition to the war in Vietnam, Humphrey represented a continuation of Johnson's Vietnam policy at the Democratic National Convention in Chicago. Violence broke out when protesters clashed with police as Humphrey accepted the nomination.
Things would be very different in 2024 if Biden decided to leave the race, although Democrats' convention will return to Chicago in August.
As we first wrote back in February, if the leading candidate was to drop out of the campaign after most primaries or even during the convention, individual delegates would need to select the party's nominee on the convention floor (or, potentially, during a virtual roll call).
That would shine a spotlight on the normally niche question of who those actual delegates are. The Democratic Party set a deadline of June 22 for states to select the more than 3,900 delegates - almost all of them currently pledged to Biden - allocated as part of the primary process.
These delegates aren't just pledged to vote for Biden; they're also approved by his campaign. So while a majority of convention delegates could decide to pick a new nominee, doing so would require massive defections from the president's own supporters. It also means that if Biden dropped out of the race, it would largely be Biden backers who would be responsible for picking his replacement.
Who could replace Biden?
You can assume, for instance, that Vice President Kamala Harris would be a top contender to be on the ballot in such a scenario. But there would be other potential candidates who previously argued they could run a more effective campaign against former President Donald Trump.
Would someone like California Gov. Gavin Newsom - who offered unqualified support for Biden in the wake of Thursday's debate - challenge Harris at the convention? Settling on a replacement could be divisive and ugly. It would be up to the delegates to decide, in a series of votes after frantic lobbying, who to pick.
On the Democratic side, there is also another group to consider: the "superdelegates," a group of about 700 senior party leaders and elected officials who are automatically delegates to the convention based on their position. Under normal party rules, they can't vote on the first ballot if they could swing the nomination, but they're free to vote on subsequent ballots.
What if a candidate left the race after the convention?
It would take a drastic event for a candidate to leave the race in the few months between a party's nominating convention in the summer and the general election in November.
Democrats and Republicans have slightly different methods of dealing with this possibility. You can imagine the end result would probably be that the running mate stepped up to be on the general election ballot, but that is not necessarily guaranteed.
Democrats - The Democratic National Committee is empowered to fill a vacancy on the national ticket after the convention under party rules, after the party chair consults with Democratic governors and congressional leadership.
Republicans - If a vacancy occurs on the Republican side, the Republican National Committee can either reconvene the national convention or select a new candidate itself.
Would the running mate automatically become the nominee?
An in-depth Congressional Research Service memo also notes that if an incumbent president becomes incapacitated after winning the party's nomination, the 25th Amendment would elevate the vice president to the presidency, but party rules would determine who rises to become the party's nominee.
Neither party, according to the CRS, requires that the presidential candidate's running mate be elevated to the top of the ticket, though that would obviously be the most likely scenario.
Has a candidate ever left the race after the convention?
In modern times, per the CRS, the Democrat running for vice president in 1972, Sen. Thomas Eagleton, was forced to step aside after the convention after it was discovered that he was treated for mental illness. (1972 was a very different time! Today, thankfully, there is not nearly the stigma attached to mental health.)
The DNC actually needed to convene a meeting to affirm Sargent Shriver as Democratic nominee George McGovern's second-choice running mate.
What if a president-elect was incapacitated after the election?
If a president-elect was to die, timing is again important.
Under the Constitution, it is electors meeting in state capitols who technically cast votes for the presidency. While some states require that they vote for the winner of the election in their state, in others they have leeway.
The CRS memo, which cites several congressional hearings on the subject, suggests it would clearly make sense for a vice president-elect to simply assume the role of president-elect, but the law itself is murky.
Under the 20th Amendment, if a president-elect dies, his or her running mate, the vice president-elect, becomes president.
There could be some question, for instance, about when exactly a person becomes president-elect. Is it after the electors meet in December, or after Congress meets to count Electoral College votes on January 6?
