What if Biden or Trump leaves the race?

WASHINGTON — Biden versus Trump: The lineup in the 2024 U.S. 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.

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.