Cain becomes a worst pick then Palin, as news of his "sexual adventures" spreads. He's forced to withdraw within a month of being picked, leaving McCain down by double digits in the polls. Faced with near certain defeat, I think it's most likely McCain picks a safe if boring conservative and tries to salvage a respectable defeat. But he might decide to try a full on hail Mary pass and pick someone like Lieberman or even Powell, if he can convince him.