First, the Republican National Committee would have to produce a new nominee, a process that would involve Chairwoman Ronna McDaniel and the 168 national members — three from each state and territory. But since many states have already started printing, mailing and accepting ballots, and some have begun in-person voting, the name of a new nominee could be unlikely to be printed on ballots in time for Election Day.

Then it would fall to individual states to decide how to proceed, and most have not set rules for this situation.

“It would be a question of what each state’s law says or doesn’t say about what happens in this eventuality, and many state laws are just silent on this possibility,” said Richard L. Hasen, a law professor at the University of California, Irvine, who also discussed the issue on his Election Law blog. “So there may be questions about what to do.”

The question would become more complex if Mr. Trump won but was unable to serve. Some but not all states bind their electors to vote for whoever wins the state, but even most states with binding elector laws make no mention of what could happen should a candidate die or be unable to serve.