What happens if a candidate dies or becomes incapacitated?
There is no Federally-required process to follow if a candidate who is projected to receive electoral votes dies or becomes incapacitated between the general election and the meeting of electors. However, individual States may have their own requirements that govern how electors must vote at the meeting of the electors. In 1872, when Horace Greeley passed away between Election Day and the meeting of electors, the electors who were slated to vote for Greeley voted for various candidates, including Greeley. The votes cast for Greeley were not counted due to a House resolution passed regarding the matter. See the full Electoral College vote counts for President and Vice President in the 1872 election.
We don’t know what would happen if a candidate who, dies after or becomes incapacitated between the meeting of electors and the counting of electoral votes in Congress.
The Constitution is silent on whether this candidate meets the definition of “President elect” or “Vice President elect.” If the candidate with a majority of the electoral votes is considered “President elect” before the counting of electoral votes in Congress, §3 of the 20th Amendment applies. That section states that the Vice President elect will become President if the President-elect dies or becomes incapacitated.
If a winning Presidential candidate dies or becomes incapacitated between the counting of electoral votes in the Congress and the inauguration, the Vice President-elect becomes President, according to §3 of the 20th Amendment.
It’s a good question. Keeping in mind that constitutionally, individual voters don’t elect the president, Electors do – here’s a partial answer. Partial because it’s mostly never happened before, so no one really knows:
From the Electoral College FAQ at the National Archive
Also see: https://people.howstuffworks.com/what-happens-presidential-candidate-dies.htm
and https://www.vox.com/21502447/trump-biden-death-what-happens (from 2020)