In order for Joe Biden to not be the 2024 Democratic presidential nominee, either he must suffer a health issue that makes it impossible for him to serve another term as president, or he must voluntarily say he does not want to seek another term.
If Biden suffers a health issue that makes it impossible for him to serve as president now, Kamala Harris becomes president. (As for the notion that Biden’s cabinet could invoke the 25th Amendment, note the amendment is binary, like an on/off switch. Biden’s cabinet can’t declare that he is competent to handle his duties until January 20, 2025, but not beyond that date.)
Right now, only Joe Biden has won delegates to the 2024 Democratic National Convention in Chicago, sweeping all of the available delegates in South Carolina and Nevada. Also note that Biden is the only candidate on the ballot in Florida and North Carolina. With Marianne Williamson withdrawn from the race, and Minnesota congressman Dean Phillips struggling to hit 5 percent nationally, it is just about certain that when the Democratic convention begins on July 15, an overwhelming majority of those delegates, and perhaps all of them, will be pledged to Biden.
Note that a Biden delegate is pledged only to Biden, and is not obligated to support Kamala Harris if Biden suddenly was no longer a candidate.
Under the Democratic National Committee’s rules, “delegates elected to the national convention pledged to a presidential candidate shall in all good conscience reflect the sentiments of those who elected them.” Nor is there any way for, say, a state party to require a delegate to vote for another candidate besides the one they’re pledged to support: “No delegate at any level of the delegate selection process shall be mandated by law or Party rule to vote contrary to that person’s presidential choice as expressed at the time the delegate is elected.”
On paper, this means the Biden delegates would have the option of nominating someone besides Harris, and state parties couldn’t require or force delegates to vote to nominate Harris.
Thus, if in the coming months Biden announced he would not serve a second term, all of those Biden delegates would be free to support any candidate they like.
In reality, it would be exceptionally politically difficult for a majority of those 4,672 delegates to skip over the first woman and first African-American vice president and nominate someone else. Harris, and her allies, would fight like mad to ensure she was the nominee, and if the delegates nominated a white male like California governor Gavin Newsom, the accusations of racism and sexism would erupt with volcanic fury. I don’t know if that course of action would tear the Democratic Party apart, but I know that Harris and her allies want Democrats to fear that course of action would tear the party apart.
Could the Democratic Party nominate someone besides Joe Biden or Kamala Harris to be their presidential nominee in 2024? Yes, but that would require two exceptionally unlikely events to occur: for Joe and Jill Biden to decide they don’t want to live in the White House for another four years, and for Kamala Harris to lose a floor flight at the convention.