I don't think another candidate could get on the ballot in all 50 states this late in the game. McCain would probably stay on the ballot and there are some morons that would probably vote for him anyway.
Happened to that weird guy that put a dress on a nude statue John Ashcroft. George Bush's Attorney General lost an election to a dead guy!
http://archives.cnn.com/2000/ALLPOLITICS/stories/11/07/senate.missouri/