The Democratic Party has no Plan B if US president Joe Biden decided for any reason to halt his 2024 re-election campaign, and a sudden need to replace him as its standard-bearer would spark a messy intraparty battle.
Despite weak poll numbers and questions, including from some Democrats, about his age, Mr Biden has stuck to his plan to seek a second term after clearing the field of serious Democratic primary challengers when he announced in April that he was running again.
Even if more Democratic candidates were to jump in now, the path forward would be unclear as deadlines to get on the primary ballot in critical states such as Nevada, South Carolina and Georgia have already passed.
Biden loyalists, citing his record in office, argue that the party does not need a backup plan to defeat probable Republican nominee Donald Trump, who Mr Biden beat in the 2020 election.
Among the possible scenarios if the president, 81, did drop out: Democrats could pick another nominee next August at their convention, or even later, in line with party rules.
Reuters spoke to multiple current and former officials who, while making clear they want Biden to succeed, acknowledge the party could face upheaval should the oldest president in US history encounter a health issue or step aside for other reasons during the White House contest.
Mr Biden's running mate, vice president Kamala Harris, who has her own popularity problems, would not automatically replace him as the top candidate if he stepped aside; she would benefit from their joint campaign infrastructure if she ran for president in his absence, but other Democrats would likely swoop in to the race as well.
There is no Plan B. If he were ... suddenly not to run, everyone you know would run. The VP scares no one.
"There is no Plan B. If he were ... suddenly not to run, everyone you know would run. The VP scares no one," a senior Democrat told Reuters.
If Mr Biden were to drop out while the Democratic primaries were ongoing, other candidates could join the race, depending on state filing requirements.
It normally takes months to set up a presidential campaign and raise the money to run, and deadlines to get on the ballot in consequential states including California, Illinois and Michigan are approaching in the coming weeks.
Democratic officials are united behind the president, despite some trepidation, they say; otherwise big name competitors already would have entered the race.
"Joe Biden will be the Democratic nominee, and he will beat whichever MAGA extremist the Republicans put forward,” Biden campaign spokesperson Daniel Wessel said, referring to former president Trump's "Make America Great Again" slogan.
Mr Trump, 77, the frontrunner for the Republican nomination, also faces concerns about his age and a litany of charges including mishandling of classified documents and interfering in the 2020 election, which he lost to Mr Biden. He denies wrongdoing.
Multiple candidates are challenging Mr Trump in the Republican primary race, giving the party built-in alternatives in the unlikely event that Mr Trump drops out.
Good of the country?
To the chagrin of the White House, Mr Biden's age has become a defining part of the 2024 campaign.
David Axelrod, a top White House adviser during Barack Obama's presidency, said Mr Biden needed to decide whether it was smart to run again, after November polling showed him lagging in key swing states against Mr Trump.
"If he continues to run, he will be the nominee of the Democratic Party. What he needs to decide is whether that is wise; whether it's in HIS best interest or the country's?" Mr Axelrod wrote on X, formerly known as Twitter.
A physical examination in February found Mr Biden healthy and "fit for duty".
Mr Biden has long believed that he is the Democrat most likely to beat Mr Trump, but a Reuters/Ipsos poll on November 7th showed his approval rating at 39 per cent, its lowest since April.
Democrats' primary elections start in February and end in June. They hold their convention in Chicago in August.
Mr Biden is expected to win his party's nomination after an easy primary process. He has a massive lead in polls over declared Democratic challengers Marianne Williamson, a writer, and US representative Dean Phillips.
If he dropped out after the last primaries in June 2024, delegates would be free to vote for another candidate in Chicago.
Any departure before the convention would almost certainly lead to jockeying among a broad group of presidential hopefuls, including Ms Harris and California governor Gavin Newsom, to convince more than 4,000 Democratic delegates to give them the job.
It would augur a return to a time in which convention delegates truly chose, not just rubber-stamped, their nominee.
"If he drops out before the convention, we will have an old-fashioned convention where the delegates essentially get to make up their mind (on whom to vote for) regardless of who they were elected to represent," said Elaine Kamarck, an elections expert, senior fellow at the Brookings Institution in Washington and a member of the Democratic National Committee.
Intraparty warfare
That could spark intraparty warfare, leaving Democrats to mirror Republicans in a battle in which candidates spend time and money fighting each other.
Things get more complicated if for any reason Mr Biden dropped out after the convention. Ms Kamarck, the author of the book "Primary Politics," said the 435 members of the DNC would then meet in a special session to select a nominee.
There is some precedent for switching out a nominee.
In 1972, then senator George Eagleton withdrew as Democratic presidential candidate George McGovern's vice presidential running mate after revelations of his treatment for depression forced him out of the race.
The DNC had an emergency meeting to fill the vacancy and selected Sargent Shriver to take his place. Mr McGovern lost the election.
Mr Newsom has been particularly active as a "surrogate" for Mr Biden by taking high-profile swipes at Republican presidential candidate Ron DeSantis, the governor of Florida. Mr Newsom's office did not respond to a request for comment.
"A bunch of the surrogate activity we see out there - like Newsom all over helping Biden - is partially to remind Democratic voters they are out there as an option," the senior Democrat said.
