The Supreme Court does leave Romney in a bit of a bind, too. Up till now, he's argued that the reason a mandate was great in Massachusetts, but terrible nation-wide, was that it was unconstitutional at the federal level. Now that the constitutionality of the measure has been reinforced, he's just left saying it's bad policy (which he said today). Which brings us back to the question of how, on a policy level, something could possibly be so awful federally that it needs to be repealed, when at the state level it was actually kinda good.
That's a good point. He could argue of course that it will cost way more than projected. But since the Massachusetts plan has proved to be more costly than projected and he has continued to defend it, he'll have to insist that our deficit woes can't hold up the national plan. Nonetheless, everyone who wants to rally behind him as the last best hope of repealing PPACA will donate and turn out, and for the rest of the swing vote out there, he will just pivot to the economy and jobs and hammer away at that. It's still going to be, I think, an incredibly close election.