What happens if Joe Biden or Donald Trump drops out of the presidential race?

Joe Biden’s faltering performance in the first 2024 US presidential debate on Thursday exacerbated voter concerns about his fitness for office.

The US president’s meandering answers and hoarse voice rekindled worries about his age — 81 — and prompted top Democrats to call for him to halt his bid for re-election.

Donald Trump, 78, also faces questions about his age, mental acuity and temperament. Next month he will be sentenced for his criminal conviction in New York.

What happens if one of them suddenly drops out of the race — or dies? The succession battle could be messy.

What if Trump or Biden drop out or die before the conventions?

Both candidates easily won their party’s primary vote earlier this year. If they dropped out now, it would be up to delegates at the forthcoming Republican and Democratic conventions to find replacements.

That would make July’s Republican convention in Milwaukee or the Democratic convention in Chicago in August akin to conventions decades ago, when candidates canvassed each state’s delegation for floor votes.

State delegates would be “uncommitted” — no longer beholden to their state’s primary result — and able to vote for any candidate they liked, said Elaine Kamarck at the Brookings Institution think-tank. It might take several rounds of voting to find a nominee.

Vice-president Kamala Harris’s approval rating is just 39.4, according to FiveThirtyEight — even lower than Biden’s 39.9 per cent. Other Democrats would almost certainly jump into the race.

Trump hasn’t yet announced his pick to be his running mate. So the Republican convention would be a “little bit more of a freewheeling world”, Muller added.

Neither party’s rules say that the running mate must succeed a presidential nominee who quits or dies.

The whole replacement process would “look messy” with “a lot of bargaining, a lot of trading favours, a lot of speeches to state delegations at conventions”, said Kamarck.

Has a presidential candidate dropped out before?

Yes, but when party primary rules were different.

Lyndon Johnson, the sitting Democratic president in 1968, shocked the nation that March by pulling out of the race, saying the presidency must not be sullied by “partisan divisions” while he focused on the contentious Vietnam war.

Robert F Kennedy, who hoped to be the nominee, was assassinated that June, leaving Hubert Humphrey to win the party’s backing at a Chicago convention marred by protests over the war. Democrats are bracing for anti-war protests — this time over Gaza — at their convention this year, again in Chicago.

Four years after LBJ’s shock withdrawal, Democrat Thomas Eagleton dropped out as the running mate of the party’s presidential nominee George McGovern. The full Democratic National Committee took control of the replacement process — setting a precedent.

These days, if either candidate is gone before the convention, the party leadership — the DNC or Republican National Committee — would step in to manage the fallout.

“The simple thing to remember is: the parties are in control up until election day,” said Kamarck.

What would happen if a candidate dropped out after the conventions?

The party committees would decide a new nominee by vote. But first there could be an almighty succession battle.

For the Democrats, vice-president Harris would start as favourite despite her own dismal approval numbers. But California governor Newsom would be a contender to replace Biden. Michigan governor Gretchen Whitmer and Illinois governor JB Pritzker are other possible runners. The problem for all of them is that they lack national name recognition, and are considered more plausible candidates for 2028.

The Republican fallout if Trump were incapacitated could be chaotic. During the primary, Republicans like Florida governor Ron DeSantis and entrepreneur Vivek Ramaswamy tried to capture Trump’s Maga supporters, and might fancy their chances. But Trump’s closest primary rival, his former US ambassador to the UN Nikki Haley, came from the more traditional conservative wing of the Republican party. So the battle to replace Trump would be an ideological fight too.

When would it get even more messy?

If either man dropped out after winning the election on November 5, but before the Electoral College voted to formally make him president-elect, things could get weird.

The electors in the Electoral College “wouldn’t be beholden to anybody”, said Kamarck. “As a matter of practical politics, they probably pick the vice-presidential nominee . . . but that’s a little ambiguous.”

Once the Electoral College has voted, it would be more straightforward: the US constitution’s 20th amendment says that if the president-elect dies, the vice-president-elect succeeds them.

Once inaugurated, that new president could nominate a vice-president under the 25th amendment. That choice would then have to be approved by a majority of both chambers of Congress.

Additional reporting by Oliver Roeder in New York

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