What would cause President Joe Biden, an 81-year-old Democrat, or ex-president Donald Trump, the 77-year-old Republican, not to be on the ballot?
Age for one. Either candidate would enter office in January 2025 as the oldest U.S. president ever.
Although neither has reported any serious health issues, statistics suggest that both face heightened risk of mortality or serious medical incident because of their advancing years.
What about a voluntary withdrawal? "It's a fabulously ridiculous fantasy," said Rachel Bitecofer, a Democratic political strategist.
Even if Republicans wanted to change their rules and switch candidates — if, for example, polls took a disastrous turn following a Trump legal conviction — "they still wouldn't, because it'd cause a total meltdown in their base."
Biden regularly asserts that he is the best qualified candidate, despite polls that indicate that his age is off-putting to voters.
FILE — U.S. President Joe Biden walks across the South Lawn of the White House, after returning from a weekend trip to Delaware, in Washington, on February 19, 2024.
"What are you supposed to say — 'Oh, he's fine, he's going to run a triathlon tomorrow'? I mean, he's 81 years old," said Democratic congressman Adam Smith. Still, "nobody of note decided to run against him, so here we are."
Trump meanwhile insists he will run despite possible criminal conviction before the election which theoretically could see him face decades in prison.
To designate a party's formal nominee, delegates from each state attend their party's summer nominating convention to officially anoint a candidate based on primary voting.
If Biden or Trump were to exit the race before the end of the primaries, final say would go to the delegates at the convention.
FILE — Former U.S. President Donald Trump attends a campaign event in Waterford Township, Michigan, on February 17, 2024.
And those delegates, Elaine Kamarck of the Brookings Institution said in a recent note, are "8,567 people you've never heard of," ordinary Americans who happen to be political active.
This has not happened since President Lyndon B. Johnson made the shock announcement on March 31, 1968 — in the middle of the Vietnam War — that he would not seek reelection.
Since then, conventions have been well-oiled affairs, whose outcomes have been known in advance since they are determined by the primaries.
But this year, the departure of a candidate could result in "a convention where the result may not be known ahead of time ... the kind of no-holds-barred event that nominating conventions held between 1831 and 1968," said Kamarck.
And what if something happens to one of the candidates after being officially nominated at the convention?
One of the parties' formal governing bodies, the Democratic National Committee or the Republican National Committee, would nominate a new candidate in an extraordinary session.
On the Republican side, the RNC is undergoing a reshuffle and Trump has suggested placing his daughter-in-law Lara in a leadership role, which would give his camp enormous influence in choosing a replacement.
A strong —but not automatic —candidate to take Biden's place would be his vice president, Kamala Harris, who is already part of his campaign.
FILE - U.S. Vice President Kamala Harris speaks next to State Senator Darrell Jackson in Columbia South Carolina, on January 15, 2024.
Otherwise, any of a number of strong Democratic politicians —Governors Gavin Newsom of California, Gretchen Whitmer of Michigan and Josh Shapiro of Pennsylvania are mentioned —might be called on.
But Trump has not yet announced his choice for vice president.
And as Hans Noel, professor of government at Georgetown University, pointed out, Trump has belittled his party's other heavy hitters throughout the primaries.
Chief among these is Florida Governor Ron DeSantis, who was relentlessly targeted by Trump before ending his candidacy.
FILE - Florida Gov. Ron DeSantis during a visit to the Florida State Fair, February 2024, in Tampa, Fla.
Also at the top of the list is Nikki Haley, the lone serious candidate apart from Trump to remain in the Republican primaries — but who is hated by Trump's loyal followers.
"Nikki Haley might have been well-positioned to be an alternative before," Noel said, but by continuing to battle against the party favorite she has lost support from "anybody who likes Trump."
Meanwhile, could a strong third-party candidate emerge? So far, no independent candidate is posing any danger to America's dominant two-party system.
In 1992, Texas billionaire Ross Perot, running as an independent, managed to win nearly 19 percent of the popular vote.
But in the end, because of the vagaries of the American electoral system, he did not receive a single one of the votes that matter most: those of the 538 members of the Electoral College that ultimately decide the winner.