The first part of the answer is that it depends on exactly when it happens; the second is that there are different rules for a serving president than for the challenger; the third is that the Democrats' and Republicans' respective rules are slightly different; and finally, individual states sometimes differ on how such a situation will be handled.
The first important date has already passed, with the last presidential primaries now concluded. All the states' delegates to the parties' national conventions therefore have a clear instruction for whom to vote.
The Democrats' national convention is from August 19 to 22 in Chicago; the Republicans' is from July 15 to 18 in Milwaukee.
If the candidate chosen in the primaries becomes unavailable before the relevant convention begins, a new candidate will have to be named there. The Republicans' and the Democrats' rules differ slightly on exactly how this process works, but the important thing is that the parties decide and that legislation or the constitution do not yet come into play.
If the candidate becomes unavailable after the national convention but before voting day, the leadership of the party concerned may still decide the matter, but without there being an opportunity to let other party members take part in the process on a large scale.
If the candidate becomes unavailable after voting day but before he officially becomes president, there is a clear provision in the US constitution: “If, at the time fixed for the beginning of the term of the President, the President elect shall have died, the Vice-President elect shall become President.”
(There is some legal uncertainty about exactly when the president's term officially begins, whether after the results of the election are announced or only after Congress has formally counted the state votes.)
In any event, things are clearer if the president becomes incapacitated after his term has already begun, and these rules also apply to the current president until his term is over: the vice president, Kamala Harris, takes over until Biden can resume his duties or until after the next election.
At this point it again becomes the problem of that team back in the White House, because the 25th amendment to the US constitution broadly provides for three scenarios:
A so-called voluntary temporary transfer, where the president is willing and able to transfer his powers to the vice president; an involuntary transfer, such as when the president is too unwell to transfer his powers; and a contested transfer, when the president disagrees that he can no longer fulfil his duties.
In all three scenarios, there is a clear process that involves, among others, the speaker of the house of representatives, the senate, the cabinet and the vice president, but none of them offer an option other than Harris taking over from Biden as president, even if only temporarily. The best possible outcome for the Democrats is therefore that Harris has to take over from Biden only after he is re-elected as president, which will give them four years of grace.
If the Democrats are indeed forced to make another plan before the election, the first question will be whether Harris should simply take over from Biden as presidential candidate. If not, will there be enough time to appoint someone else, and who might this be?
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