There are limits in politics. You can get away with changing a position—perhaps Romney really did see the light on abortion, not just the results of an Iowa focus group—but you can't just reinvent yourself out of whole cloth. You can go negative on your opponents, but it's a stretch to attack them for taking the same positions—on immigration, most notably—that you used to take, especially when you keep getting caught having illegals tend your garden. The sheer cynicism is driving Romney's Republican opponents nuts. He is wildly unpopular among his peers. "I just hate the guy," says a rival campaign manager. "If we can't win, I want to be sure he loses."
I expect these hard feelings to be a pretty big factor in the race.