So let's say that the individual mandate gets overturned this morning, as well as other related provisions - but the rest of the law stays.
President Obama will be hard pressed to immediately wage a fierce campaign defending the remaining of the health care bill. Mitt Romney, on the other hand, won't (or shouldn't) lift a finger.
Apart from spending a few days mentioning that Obama spent a lion's share of his first term on a piece of legislation that got swept out from under it - Romney shouldn't harbor too much on the subject. For independents and conservatives, a Supreme Court ruling will simply codify their arguments (in their eyes), so what more needs to be said.
Two things can happen from there:
1.) Democrats and the press will want to know what Romney will do about health care - which will be an even more interesting (devastating) answer, considering the model of his own health care plan was deemed unconstitutional on the federal level. And I sure hope he doesn't think he can get away with avoiding policy positions forever...especially when the debates start up.
2.) Romney will gently say: "You see, this legislation went against the nation's constitution. But that's in the past. Now let me tell you more about jobs." If this is the case, and the Obama campaign (or at least Democrats in Congress) brings out its rage on the SC, then Romney will probably look like the higher figure.
But hey, who's to say.