Republican nomination battles used to be punch-out affairs -- win enough early primaries, and your opponent would be forced to quit because he couldn't raise enough money. But in the era of super-PACs, Newt Gingrich and Rick Santorum can stay in the race as long as their billionaire patrons, Sheldon Adelson and Foster Friess, keep writing checks. Ron Paul, meanwhile, has an army of idealistic supporters who pony up $50 each week that he keeps campaigning.
The three challengers all have good reason to stay in:
