Stated Justice O'Connor for the Court in New York v. United States, 505 US 144 (1992):

"While Congress has substantial powers to govern the Nation directly, including in areas of intimate concern to the States, the Constitution has never been understood to confer upon Congress the ability to require the States to govern according to Congress' instructions. See Coyle v. Oklahoma, 221 U.S. 559, 565 (1911). The Court has been explicit about this distinction. "Both the States and the United States existed before the Constitution. The people, through that instrument, established a more perfect union by substituting a national government, acting, with ample power, directly upon the citizens, instead of the Confederate government, which acted with powers, greatly restricted, only upon the States." Lane County v. Oregon, 7 Wall., at 76 (emphasis added)..."

..."The inadequacy of this [Confederate] governmental structure was responsible in part for the Constitutional Convention. Alexander Hamilton observed: "The great and radical vice in the construction of the existing Confederation is in the principle of legislation for states or governments, in their corporate or collective capacities, and as contradistinguished from the individuals of whom they consist." The Federalist No. 15, p. 108 (C. Rossiter ed. 1961). As Hamilton saw it, "we must resolve to incorporate into our plan those ingredients which may be considered as forming the characteristic difference between a league and a government; we must extend the authority of the Union to the persons of the citizens--the only proper objects of government." Id., at 109. The new National Government "must carry its agency to the persons of the citizens. It must stand in need of no intermediate legislations . . .The government of the Union, like that of each State, must be able to address itself immediately to the hopes and fears of individuals." Id., No. 16, p. 116."

..."In providing for a stronger central government, therefore, the Framers explicitly chose a Constitution that confers upon Congress the power to regulate individuals, not States. As we have seen, the Court has consistently respected this choice. We have always understood that even where Congress has the authority under the Constitution to pass laws requiring or prohibiting certain acts, it lacks the power directly to compel the States to require or prohibit those acts. E. g., FERC v. Mississippi, 456 U. S., at 762-766; Hodel v. Virginia Surface Mining & Reclamation Assn., Inc., 452 U. S., at 288-289; Lane County v. Oregon, 7 Wall., at 76. The allocation of power contained in the Commerce Clause, for example, authorizes Congress to regulate interstate commerce directly; it does not authorize Congress to regulate state governments' regulation of interstate commerce."