Biden isn't leaving the 2024 race, but how would Democrats pick a nominee if he did?

Republican presidential candidate Nikki Haley recently suggested, without evidence, that President Joe Biden will soon be leaving the 2024 race in the wake of a scathing new report from prosecutor Robert Hur that described Biden has having notable memory issues, which he denies.

"You can't tell me that everybody in the Democrat Party is not trying to figure out what they do," Haley claimed on the trail on Monday.

The president has forcefully defended his record and fitness for office, and no high-profile Democrat has called for him to end his reelection campaign, with the vast majority of the party having rallied behind him.

"[Donald] Trump is going on 78, and the president is 81 years old ... It's the same. They are old, older folks, that are our choices, and that's what this nation wants," Pennsylvania Sen. John Fetterman told reporters on a Friday call organized by Biden's campaign. He called Hur's account of Biden's memory a "smear."

But if the president were to, hypothetically, withdraw from the race or find himself unable to run due to a health-related crisis, high-level Democrats could not simply find someone else to replace him at this point in the election cycle, as Haley or other observers have suggested, according to experts and party rules.

Depending on exactly when Biden were to withdraw from the race, in this scenario, his replacement would first be up to primary voters or, later, Democratic convention delegates or party members themselves.

"It depends on when this disaster happens," Elaine Kamarck, a senior fellow at the Brookings Institution and member of the Democratic National Committee, told ABC News.

If Biden were to die "in the middle of June when all the primaries are over, then it is only the delegates that get to decide," Kamarck said. But if Biden were to die "tomorrow, there would probably be some primaries that people would run in and get their own delegates elected. It's a timing question."

The president's physician has said that he "remains fit for duty, and fully executes all of his responsibilities without any exemptions or accommodations."

While Vice President Kamala Harris would assume the presidency if Biden were to have to leave office, she would not automatically inherit his place in the 2024 race or the delegates he may have won to that point, which technically award the presidential nomination at the party's national convention in August.

Harris, like any new candidate, would need to mount a campaign herself, though she would be seen as having a key advantage given her standing.

What happens if Biden leaves the race right now?

If Biden hypothetically ended his campaign now, in mid-February, Democrats would be in a bind trying to find another popular alternative.

Only one relatively prominent other Democrat remains in the nominating race: Minnesota Rep. Dean Phillips, though Phillips has had some trouble qualifying for the ballot in certain states and has largely been rejected so far by the party establishment and primary voters.

So if Biden were to withdraw from the race, it's exceedingly likely that a new Democratic candidate (or multiple) would then look to jump into the race.

Those new candidates would face a massive hurdle, however: They would have no way to secure a delegate majority at the ballot box -- which is needed to clinch the party's 2024 presidential nomination -- because they would not appear on the ballot in most states, as many filing deadlines have already passed.

As of Feb. 13, a candidate could still potentially only get on the ballot in 11 states and territories -- worth a combined 485 pledged delegates out of 3,936 total, per ABC News' data.

Still, there would be a few possibilities for a new challenger at this stage to try to win some primaries and delegates.

There's a chance that in the event of a crisis in which Biden ended his bid, the DNC would file lawsuits seeking filing deadline extensions in some primaries and some state legislatures might even reschedule their deadlines or their entire primary elections to accommodate potential new candidates.

New Democratic candidates could also attempt to mount write-in campaigns in key states where the filing period has already closed, but not all states allow write-in options.

But it would nonetheless be very difficult for any candidate to clinch a majority of the 3,936 delegates in a scenario in which Biden left the race in February, meaning that the nominee would likely have to be decided at the Democratic National Convention -- something that has never happened in the modern nominating system that started in the '70s.

From March until June

The majority of high-profile, delegate-rich state primary elections will have concluded by early March, after Super Tuesday on March 5, in which 15 states vote at once in the Democratic race, including California and Texas.

The period between then and June could be one of the most uncertain times for Biden to withdraw, according to experts.

Biden will have presumably already won close to a majority of the total pledged delegates after Super Tuesday, based on the polling and his performance in early states so far.

There would still be many smaller primaries left to go but a new candidate would have missed out the filing deadlines, as described above -- so even if they were able to mount a write-in campaign in some places, they almost certainly wouldn't be able to earn the majority-delegate share that would make them Biden's replacement.

Thus, no one would go to the Democratic National Convention (which takes place Aug. 19-22 in Chicago) as the presumptive nominee.

Subject to a DNC rule change, the delegates who were won by Biden before he hypothetically left the race would likely go into the convention uncommitted, according to Kamarck.

Democratic delegates are "pledged" to a candidate based on how they do with voters, and while party rules say that delegates "shall in all good conscience" reflect the views of those who elected them, there is no penalty if a delegate votes for a different nominee.

Any new candidate who wanted to run in the event that Biden ended his campaign, and no one else had won a majority of the delegates, would have to get nominated at the convention itself.

In that case, new candidates would first need to get at least 300 delegate signatures and then they would need to win over a majority of the 3,936 total delegate at the convention in order to become the nominee.

The specific delegates are selected either through the primaries or, more often, at congressional district and statewide caucus-conventions or by party committees.

In a scenario that Biden were out of the race, there could be a "mad rush" of people seeking delegate positions so that they can play kingmaker at the summer convention, Kamarck said, because there would be a scramble by other new candidates to win their support.

Should Biden have to bow out, Harris -- as Biden's running mate and, presumably, the sitting president -- would be a natural, even likely, destination for many Democratic delegates.

From the conventions to when ballots are printed

Only if Biden left the race after the convention, when he is expected to receive his party's nomination, would the DNC then inherit the power to choose his replacement.

Chairman Jamie Harrison would confer with Democratic leadership in Congress and the Democratic Governors Association and would then take the decision to the DNC, according to the party's call to convention.

The 483 members of the DNC -- who comprise the chairs and vice chairs of each state Democratic Party committee as well as members elected from all 56 states and territories, plus Democrats overseas -- would vote on a new nominee.

There are no rules governing who that person has to be; the nomination would not, for instance, just go to the former nominee's running mate or the person who won the second-most delegates in the primaries. They just need to get a majority of party members to vote for them.

ABC News' Oren Oppenheim and 538's Geoffrey Skelley contributed to this report.