[antognini]

Well, not quite. The Constitution is set up to delimit the powers of the federal government. As the 10th amendment states, "The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people." So if a particular law exceeds the power granted to Congress by the Constitution, it is unconstitutional. It's not that the Constitution prohibits Congress from doing certain things and everything else is by default allowed. The Constitution allows Congress to do certain things and everything else is by default not allowed.