Even if you accept at face value Romney's assertion that what's good for the state is not good for the nation, even that argument fails.
Opponents of the mandate are claiming that it violates their constitutional rights. Well, the state of Alabama, for example, has no more right to violate your constitutional rights than has the United States. Otherwise, bus stations in Alabama would still have "white" and "colored" sections.
This was a bogus argument all along.
If anything, the United States has as much right to impose a mandate as Massachusetts, because it has the right to regulate interstate commerce. The states have the right to regulate intrastate commerce, but if Romney claims that as the reason Massachusetts could do it, then they have admitted that in fact it is commerce.
And it is commerce. Opponents have been focusing on the wrong thing. The government is not regulating the buying and selling of insurance - you are not even required to buy insurance, for heaven's sake. You have an option to pay a penalty (tax?) instead. Not having to pay a penalty because you bought insurance is like getting tax preference for buying an electric car. Nobody made you do it, but you aren't going to get the tax preference unless you do.
No, the government is not regulating the buying and selling of insurance with the mandate. The government is regulating the buying and selling of healthcare.
Now if you want to make the argument that not everyone buys healthcare, try it. It will fail. They may not PAY for it, but they buy it. Commerce takes place in the transaction for the service, not in the payment of the bill.
You could argue that not everyone buys healthcare, and that's true, but only in the fugitive sense. For all practical purposes all Americans use healthcare one way or another.
You could also argue that not everyone ships stuff from one state to another, but that doesn't mean interstate shipping is not commerce.
That is the position that the Obama administration must take if it wants to win this fight.
As for Romney, when he attacks Obama over the mandate, he's pissing in the wind.