To add to Tony's response, this is a line from the US Constitution:


In particular, that is the Tenth Amendment, the last of the "Bill of Rights" that was passed at the same time the Constitution was accepted. It was a necessity to get the Constitution accepted, as the United States back then operated far more like the European Union does today, with the incorporated political entities being fairly independent.

Republicans often use that amendment as a basis for not overreaching the power of the federal government. That is not to say that they don't ignore it when it suits them, or that they would be more in favor of increased government at the state level. It's merely a cudgel. That said, it is what the Constitution says, and there has been a lot of legitimate debate about expanding federal powers by stretching the notion of other elements of the Constitution fairly thin, especially the Commerce Clause, which, to be fair, is the Democrats' cudgel.