President Joe Biden’s debate performance, which was widely panned including by some Democrats, has led some in his party to fret about whether the 81-year-old can defeat the expected Republican nominee, former President Donald Trump, in November.

Meanwhile, though Trump’s own status as presumptive nominee remains strong within his party, his pending sentencing on a 34-count felony conviction, and his age of 78, have raised similar questions among critics.

What would happen if either candidate, for whatever reason, is unable to run as the nominee?

In an updated version of an article we published in February, we’ll look at a rundown of several scenarios.

What if the presumptive nominee gives up the nomination before the start of convention?

Biden has shown no inclination to step down, either before or after the debate. At a Raleigh, North Carolina, event the day after the debate, Biden roared to supporters, “When you get knocked down, you get back up!” And leading party figures, notably former President Barack Obama, have expressed confidence in Biden as the party’s presumptive nominee.

Unless a wide swath of party leaders comes out publicly against Biden, the likelihood of a switch in nominees seems unlikely, despite the panic within some Democratic circles, political experts said.

If it did happen, much would hinge on timing, especially around the party conventions. Republicans will hold their convention in Milwaukee from July 15-18; the Democrats will hold theirs in Chicago from Aug. 19-22.

Biden could forgo the nomination voluntarily. Although dislodging him against his will is theoretically possible, it’s much less likely, because the process of choosing delegates means that the ones “ultimately selected are quite loyal” to Biden, said Josh Putnam, a political scientist specializing in delegate selection rules and founder of the political consulting firm FHQ Strategies LLC.

Forcing Biden out of the nomination would depend on delegates using a party rule that says they can fail to back the candidate they represent “in all good conscience.”

In the scenario that Biden cedes the nomination on his own, the delegates allocated during the primary season would decide the successor. Because these delegates are almost entirely pledged to him, any Biden endorsement of a nominee would carry significant weight, though even pledged Biden delegates could choose to spurn his choice.

One natural choice for a Biden endorsement would be Vice President Kamala Harris. But at this stage of the campaign, Harris would not automatically become the presidential nominee. The delegates would have to agree to that, and Harris might face competition for the nomination from other leading Democrats, such as prominent governors or senators. (If Harris was chosen as the presidential nominee, there would need to be a separate contest to select the vice presidential nominee.)

If there is a contest for the presidential nomination, it would initially play out within the 4,000 regular delegates. If one candidate secured a majority, he or she would get the nomination. If it went to a second ballot, another group of delegates — party leaders and elected officials known as “superdelegates” — could weigh in and perhaps break the deadlock.

Because running mates can’t hail from the same state, Gavin Newsom, the Democratic governor of California, would face an obstacle if he were to share the ticket with Harris. Both come from California, and that poses a similar problem with securing electoral college votes as Sen. Marco Rubio, R-Fla., would have if he were to be tapped as the ticketmate for Trump, a fellow Floridian.

The Republican process would be broadly similar to that for the Democrats, but given the strength of Trump’s support within the party, the possibility of him being forced off the ticket is considered even more remote.

Either way, any vacancy that leaves an unsettled convention, rather than one that’s carefully choreographed, as has been standard in recent decades, could become a political show for the ages, with furious backroom lobbying to secure delegate support.

One quirk this year is that, given concerns about an early deadline for ballot access in Ohio, Democrats are planning to formally nominate their candidate before the convention opens, rather than at the convention, where a roll call of the state delegations is typically a highlight. Though details of this advance nominating process are not finalized, that’s supposed to happen by Aug. 7, or less than six weeks after the June 27 debate.

What if the presumptive nominee is nominated at the convention but quits the presidential race before Election Day?

If Biden or Trump is approved as the nominee at their respective conventions but has to leave the ticket before Election Day on Nov. 5, the rules of succession would be different.

First, if Biden were to die in office (or be made to relinquish the presidency because of incapacity under the 25th Amendment), Harris would become the incumbent president. Harris would also become president immediately if Biden were to simply resign as president.

What would this mean for the presidential nomination? On the Democratic side, the rules would empower the Democratic National Committee to name a successor. While Harris would not automatically become the nominee, that would be considered the most likely option, assuming she wanted the nod.

If Harris were elevated to presidential nominee, the same process would determine the new vice presidential nominee. This happened in 1972, when the DNC named Sargent Shriver as the vice presidential nominee after Missouri Sen. Thomas Eagleton left the ticket following revelations about his mental health.

The Republican rules are murkier. A reconvening of the national convention is possible, but the Republican National Committee could probably find an alternative mechanism, Putnam said. Regardless, “it would not be as clean or as clear a move as the process on the Democratic side,” Putnam said.

What happens between the election of a president and their inauguration?

If a new president is elected but dies before inauguration, the duly elected vice president would become the next president. A new vice president to serve alongside the newly elevated president would need to be approved by the Senate and the House, as happened when Congress approved Nelson Rockefeller after Gerald Ford took over for Richard Nixon, who had resigned amid the Watergate scandal.

There’s still room for more wackiness. According to The Washington Post, if the winner dies between when the electoral votes are cast and when Congress counts them Jan. 6, 2025, it’s unclear what would happen, even for the National Archives and Records Administration, whose job it is to know.

“We don’t know what would happen” in that scenario, the agency says on its website.

PolitiFact Staff Writer Marta Campabadal Graus contributed to this report.

This article was originally published by PolitiFact, which is part of the Poynter Institute. See the sources here.