WASHINGTON, Feb 18, 2024 (BSS/AFP) - Biden versus Trump: The lineup in the 
2024 US election has long been a foregone conclusion, with a rematch between 
the two presidents appearing all but certain.

But what would happen if, for any number of reasons, one of the two 
contenders does not end up taking part in the November vote? Apart from 
immense political confusion, here is what would happen, according to the 
rules and experts:

- Why not on the ballot? -

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 US 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.

"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.

- If a candidate leaves -

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.

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.

- Who might fill in? -

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.

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 VP.

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.

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.