Darn, the board just ate my elegant and charmingly phrased question on this subject, I’ll have to start over : what – if any – procedures are in place to cover the death of a candidate at various stages of the Presidential nominating process? Up until a certain point it would simply be “dropping out of the race with extreme prejudice”, one presumes, but as the delegate count begins to mount, where do the suddenly-released delegates go? What happens if the deceased had effectively clinched the party’s nomination first? What happens if the deceased pops off once he’s officially the Party’s nominee (as actually happened in the last two election cycles at the Senate level with Mel Carnahan and then Paul Wellstone) – and what are the various make-or-break dates involved for replacing him? (IE the kerfuffle of Wellstone’s death falling just on the “must name new candidate” side of the date whereas Carnahan was already on the ballot and the Gov had to specify a replacement-presumptive)
The Master Speaks: What if the U.S. president and vice president-elect die before being sworn in?
Cecil answers at least part of your inquiry.
In many states, part of the nominating process is the naming of what’s called a “vacancy committee” – people not including the candidate who are supportive of the candidate and his stance on issues, who will get together and agree on a replacement if he dies, has a massive stroke or heart attack, is sent to prison for molesting a four-year-old, or otherwise ends up unable to serve. The person they agree on replaces the disqualified candidate on the ballot.
Vacancy committee membership is usually honorary, but does have that long-chance responsibility.
If a presidential nominee died after the nomination but before the election, the party would just get together, reconvene, and pick a new guy. If it were before the convention, the same rules would apply.
If the candidate died after it was time to change the ballot in some states, presumably the dead candidate would stay on the ballot and if that state voted for the dead guy, the electors would vote for the replacement.
This is my speculation however.