In an interview with Bill O'Reilly, Romney made an interesting argument supporting mandates at the state level versus mandates made at the federal level.
Romney said that even though people may not realize it, the state government has lots of mandates on citizens, mandates that would be unconstitutional for the federal government to do. For example, state governments mandate that all kids go to school. The federal government can't mandate that all kids go to school because the federal constitution doesn't give the federal gov. that power.
I don't think most people realize that each state has its own constitution, and that the federal government also has its constitution. Too often when we talk about "the constitution," we only think of the federal constitution. But it is important to remember that each state has its own constitution as well, and that it is the state constitution that gives the state different powers than the federal government. The powers of the state constitution are much more broad and encompassing because it is meant to fill in the gaps where the federal constitution has been prohibited to go. The founding fathers wanted a limited federal government and as such, they gave it a very limited and narrow set of powers. It is the state governments that fill in the gaps with their own unique constitutions.
Also, state governments mandate that drivers must have auto insurance. The federal government can't do that either. Romney goes on to say that some people counter that auto insurance is only required if you have a car. And Mitt responds to that by saying "Well what state do you live in where you don't need a car?"