Easy, he says "It was the state of Massachusetts choice, the people of the state have the sovereign right to make that decision. The Federal government, on the other hand does not have that right to decide "FOR" the people. What else can they decide is "FOR" your best interests and constitutional? Gun haters, the US Constitution states that Defense is the responsibility of the Federal government. The SC ruling, today, just laid the ground work for some future government to say "It is in the best interests of national defense for everyone to have a gun. You don't have to buy one; if you don't though, we will tax you. The same can be said for going to a church, of any type. It is in the general welfare of the people that they be happy. Since people of faith tend to be more happy and content, everyone has to now attend some sort of church/religious service; if you don't, you will be taxed."
Sure that can work in long form thought, but in a sound byte election that will be hard to pull off. Though I will say Romney has amazing talent to spin on his feet. If anyone can pull it off it is him, compared to McCain, Dole or either Bush.