https://abcnews.go.com/538/biden-trump-suddenly-leaves-2024-race/story?id=106136493

From the conventions to when ballots are printed

The national conventions are a key turning point in our hypothetical calendar. Before them, primary voters, or delegates selected through the primary process, would still have the ability to choose their party's nominee. After the conventions, though, the Democratic and Republican national committees would inherit that power.

Both the DNC and the RNC have enshrined in their rules a process for how to fill a vacancy on the party's ticket after the formal nomination has already taken place. For Democrats, there is only one option: Chairman Jamie Harrison would confer with Democratic leadership in Congress and the Democratic Governors Association and would then take the decision to the DNC, according to the party's call to convention.

The 483 members of the DNC — who comprise the chairs and vice chairs of each state Democratic Party committee as well as members elected from all 56 states and territories, plus Democrats Abroad — would vote on a new nominee. There are no rules governing who the nominee has to be; the nomination would not, for instance, just go to the former nominee's running mate or the person who won the second-most delegates in the primaries. They just need to get a majority of party members to vote for them.

Experts say that could be a political mess, with various factions of the party pressuring members to choose one nominee or another. "They would have all sorts of internal politicking. There would be competition between various factions within the party," Richard Pildes, a professor of constitutional law at New York University Law School, told 538.

For their part, Republicans have two options for filling a vacancy, according to the party's rules. Like the Democrats, they could choose to have their committee members vote. There are three RNC members per state and territory, but they get to cast the same number of votes their state or territory's delegation was entitled to cast during the Republican National Convention. If members of a delegation aren't in agreement on who to support, their state or territory's votes would be divided equally among them. In order to become the nominee, a candidate must secure a majority of votes.

But the RNC is also "authorized and empowered to fill any and all vacancies" by reconvening the national convention.

In either case, the results of all of the primaries and caucuses would no longer formally matter. While the primary results would be one source of information for the members (if they vote) or delegates (if they reconvene the convention), they wouldn't be bound to choose the person who came in second in the primaries. They don't even have to choose somebody who ran in the primary.

Beyond their distinct rules, Pildes did not think there would be much difference between how Democrats and Republicans would deal with a candidate's death. The RNC is much smaller than the DNC, which could have an impact. "It's always easier to reach decisions in a smaller body than a larger body, and so that might be a significant difference in the way the two parties are governed," Pildes said. "But other than that, I don't think there's a dramatic difference."

[continued in child]

They’d pick someone else, probably whoever was the #2 candidate or the VP pick (with them choosing a new VP, since the election hasn’t happened yet). The electoral college can pick whoever it wants, so as long as the party has consensus I don’t see it being a huge deal.

Nikki Haley stood down her campaign but didn't actually conceed. she's still technically in the running for president. so i imagine it would default to her

A lot of it depends on timing.

There's a really good fictional take from before Bush/Gore, called "The People's Choice" by Jeff Greenfield of all people.

https://en.m.wikipedia.org/wiki/The_People%27s_Choice_(novel)

Candidate wins the election, but dies before the electoral college votes.

So... the VP guy wins? Yeah, no. He wasn't the candidate and can't assume the role of President because he was never affirmed as Vice President.

So... the other candidate wins? Yeah, no. They didn't get enough electoral college votes to be President.

So it all comes down to faithful and faithless electors.

Then the republican party would pick someone else.