First of all, Obamacare is unpopular (a fact a lot of political coverage conveniently overlooks). That's why Democrats don't talk about it on the stump, and neither did Obama for a very long time -- until the Supreme Court forced him to re-acquire political ownership. If the court upholds the Patient Protection and Affordable Care Act, Obama is not going to want to remind voters of his responsibility for an unpopular piece of legislation. If the court throws it out, Obama is not going to want to remind voters that his signature accomplishment -- which distracted him from a bad economy and cost the Democrats the House -- was so flawed that the court had to reject it. Either way, Obama will not be racing to talk about health care. But Romney will.
It's often said that Obama will respond to Romney's attacks by saying the mandate was based on Romneycare. Romney will respond, "Well, you did it wrong" and promise to repeal and replace the law. All the voters really need to know is that Romney is against Obamacare. Besides, Romney will have plenty of other lines of attack: the raid on Medicare, the rationing board, the tax hikes, the religious liberty issues, the creation of a vast new entitlement when the existing ones are crushing us with debt, etc.
Core Republican voters will vote against Obama, not for Romney. Polls show GOPers are more enthusiastic about voting in 2012 than Democrats. Meanwhile, the independents and moderates who dislike Obamacare, but who are not libertarians, will most likely see Romneycare as evidence that Romney is not one of the right-wing crazies the "Today" show keeps warning them about.
Democrats are truly comfortable only attacking Republicans as "extremists" of one flavor or another. But over the weekend Joe Biden tried attacking Romney for being too "flexible." That might have bite coming from the right in the primaries, but it looks as if the primaries are over.