A candidate need not run in order to obtain the office. See Gerald Ford. Votors vote for electors in NOvember. Electors vote for President in December. Electors vote for the person their party tells them to vote for.
The law further contemplates unavailability of President, President-elect, etc. at ANY time, for ANY reason.
One of these days, we are bound to have an election that is unsettling to the voters.
I have this sinking feeling that it's going to be THIS year. As PD said months ago, it will be Edwards/Hillary. Probably at the last minute, after the debates. Even Swift Boat Veteran O'Neill said he would vote for Edwards, and after the F'n campaign fiasco, the Dim base will be all fired up on election day.
We already had one in 2000. This one is likely to be worse since so many states have electronic voting with no paper backup. This makes recounts impossible and any close state will be Florida 2000 squared. There will be several close states.