Like everything else the last two years, the GOP opposed the mandate on the grounds that Obama advocated it. Can't help but notice how many Republican polls are led by the father of the mandate. If Romney's campaign is sunk it won't be because Republicans oppose the mandate as much as the Republicans oppose things they're told are similar to Obama. Just like Charlie Crist didn't lose the primary for supporting a stimulus plan during a labor crisis that was 40% tax cuts. He lost because he hugged Obama.
Do you really think the ideological extent of the GOP is "Do what ever Obama doesn't do, and vice versa"? Conservatism has been around longer than he has you know.