If something happens to a nominated candidate before the election, what does the Constitution say occurs?
The Constitution does not deal with political parties. Political parties are not even a part of our government. Lou Dobbs made quite a point of this when both the Democratic Party and the Republican Party penalized states who held primaries early by not counting their delegates. If something happened to a candidate a party nominated, the party would have to run another candidate. It would be interesting to see how either party would deal with that. I think it would be most likely that they would run the candidate that was next runner up. Hillary would likely have replaced Obama and maybe Romney would have replaced McCain.