By March 20, 2024, Donald Trump will have amassed sufficient Republican delegates to clinch the Republican presidential nomination. On the same date, Joe Biden will have accumulated the delegates required to secure the Democrat nomination. Thirty-three weeks before the election, the nominating process will theoretically be concluded.
However, Joe Biden will be the oldest presidential nominee in American history (he will be 82 in November) and suffering from noticeably accelerating mental and physical disabilities. If over the next seven months he were to die, or have a debilitating stroke or, in a far more likely scenario, is forced by the party hierarchy (i.e., Barack Obama) to drop out of the race due to his age, escalating infirmities and persistent unpopularity, the process to replace him is relatively straightforward.
The only considerations will be when Biden exits the race and the timeframe necessary to print ballots. However, choosing someone to replace him is a potential minefield only Barack Obama can navigate.
Per Elaine Kamarck of the Brookings Institution, if Biden were to leave the race any time after March 19th and before mid-June, those delegates committed to him would likely go to the convention uncommitted.
However, the party could adopt a new rule in conjunction with the rules of the individual state parties which would allow delegates to switch to other candidates prior to the convention. But that highly unlikely process could eventuate in potential chaos and thwart the ability of the Obama cabal, who control the party and the White House, to determine the Democrat nominee.
If Biden were to leave the race after the primaries are over in June and before the convention, it would be impossible to adopt rule changes and the nomination will then be decided at the convention in August.
Democrat delegates are “pledged” to a candidate based on the results of the primaries, and while the DNC rules say that delegates “shall in good conscience” reflect the outcome chosen by the voters who elected them, there is no penalty if a delegate votes for a different nominee at the convention. Thus, they are essentially not required to vote for a particular candidate.