I would think that there would be time constraints and how much time Obama has to change his pick for Vice President with the various paper ballots and all the bureaucracy involved with the November 2008 election. Does not anyone remember the hanging chads?

If a Vice Presidential pick gets seriously ill or otherwise incapicitated I can understand changing your horse when you are almost at the other bank but this would really confuse a lot of voters if a Presidential candidate can just write someone else in at the last minute??