Primary Season Getting Earlier - washingtonpost.com

What happens if and when something happens to the nominee. Since the gap between the primaries and the nominating conventions are spreading out. I wonder if we have a nominee by 5 February; what happens if there is some sort of scandal (ala Ryan in Senator's race in IL) or some sort of medical tragedy. What's the contingency plan? Do you renominate someone? Go through the primaries again? Pick who came in second? Smoky back room decision? It would be good to know this.