Election uncertainty: What if Biden or Trump can't continue their 2024 run?

United States President Joe Biden and former President Donald Trump are two of the oldest candidates making a run for the White House. Trump turned 78 years old last month, and Biden will turn 82 shortly after election day in November.

They are also two of the most unpopular presidential candidates in American history. Following last week's debate, critics are calling on both men to step down so other contenders can vie for the presidency in November.

So what happens if Trump or Biden somehow become incapacitated, die or decide to withdraw from the presidential election? Before delving into the potential scenarios and the uncertainty they would create, here's what likely won't change:

According to the US Constitution, Biden as the sitting president will remain in office until noon on January 20, 2025 (inauguration day). If he is somehow unable to perform his duties until then, the Constitution states that his Vice President, Kamala Harris, would assume the role.

Additionally, federal law ensures that key election-related dates cannot easily be changed. Election Day is set as the Tuesday after the first Monday in November. The electoral college meets the first Monday after the second Wednesday in December to cast its votes for president and vice president, based on state voting totals. And Congress officially certifies the election on Jan. 6.

But what if something happens to Biden or Trump during the election process? The Constitution, party conventions, state and federal laws as well as precedent would all weigh in.

According to experts, timing is everything. Here's what to know if something happens:

Before the nominating conventions

Both Biden and Trump have already secured enough delegates from primary voting to be their parties’ nominee. This is expected to be formalised at the Republican National Convention in Milwaukee, Wisconsin in July, and the Democratic National Convention in Chicago, Illinois, in August.

If something happens before the two parties pick their candidate, then delegates are largely free to come up with an alternative choice. Expect there to be a lot of backdoor lobbying if this is the case.

Though if for example Biden decided not to run for president, he could give a speech endorsing someone else. "That would not bind the delegates but would obviously carry a lot of weight," Elaine Kamarck, a senior fellow at the Brookings Institution and an expert in the party's nominating process, told ABC News.

After the conventions but before November

This is where it gets tricky. If Biden or Trump were to exit the race after the summer but before election day, both Democrats and Republicans would task their national committees to select a replacement.

The committee is composed of party insiders, with representation from each state. They would meet and vote under the rules stipulated by their respective parties.

Notably, the nomination would not automatically go to the vice presidential running mate, though that might be the easy choice for the incumbent party, said Ben Waterhouse, associate professor of history at the University of North Carolina at Chapel Hill.

Speaking to TRT World, he added, "The fuzzy part is, if it's the challenger, if Trump dies, then the Republican party has to figure out if it has time to nominate someone else, and how to do that."

There is some precedent for replacing a candidate after the convention.

In 1972, the Democratic vice-presidential candidate, Senator Thomas Eagleton, was nominated alongside Democratic presidential candidate George McGovern at the national convention. But VP pick Eagleton was forced to withdraw from the ticket after reports emerged that he had endured electroshock treatment for depression.

The Democratic National Committee then convened a meeting in Washington to select Eagleton’s replacement, Sargent Shriver.

More recently, when Access Hollywood tapes emerged in October 2016 of Trump bragging about sexually assaulting women, many Republicans discussed dropping him from the ticket, Waterhouse said.

"The problem at that point is that individual states had already printed their election ballots. At a minimum you would have people going into the election and voting for (someone whose name is not on the ballot). No one knows how this would work."

After election day

In the US, Americans don't vote for a candidate directly. The Constitution instructs voters to cast ballots for slates of electors. These 538 electors, who are mostly picked at state party conventions, then meet in December to vote for the president and vice president.

Electors' votes almost always reflect how their state voted, though there were "fake electors" schemes in at least six states in 2020. Experts say that criminal investigations into these plans, which involved casting votes for Trump even in states where Biden was declared the winner, appear to have dampened enthusiasm for pursuing this type of fraud again.

After the electoral college votes, the results must be counted and certified by the newly elected Congress on Jan 6. The traditionally ceremonial affair was disturbed in 2020 following the US Capitol riots which attempted to prevent the certification of Biden's victory.

If the winning candidate were to die before either of these events happened, they would likely support a replacement candidate chosen by the party.

This may or may not be the running mate. Another option is for the electors to back the deceased candidate, which means under the 20th amendment that the vice-president-elect would become the president-elect, the Guardian reports.

After Jan. 6

Once Congress certifies the election, it "casts in stone the person who will be president on January 20," Waterhouse said, adding that this means if something happened to the president, the vice president would take office.

"What is unclear is what happens if the president and vice president couldn't take office. At that point you'd have a constitutional crisis," Waterhouse said.

But given that this window of potential uncertainty is so short, such a scenario is unlikely.

Ultimately, while a candidate dying or withdrawing abruptly from the race would generate many questions, experts said they don't believe it would destabilise the election due to already existing guidance for such events.

Speaking to ABC News, Lara Brown, a political scientist and author of "Jockeying for the American Presidency: The Political Opportunism of Aspirants," said that it "doesn't mean that if we went to the contingency plan, that our system is lost. We're just in plan B — and plan B usually isn't optimal. But it is serviceable."