They can’t prevent enforcement---SCOTUS will quash it.

But while states are not powerless in the face of federal law, there are limits to what they can do to prevent enforcement of constitutionally valid regulation.

The bills are based on the theory of nullification, which has its roots in the late 18th and early 19th centuries and holds that the federal government exists by the will of the states, and that states therefore have the right to decide which federal laws are constitutionally valid within their borders.

When it comes to gun control, the claims of nullification advocates are threefold: no state is required to enforce federal gun regulations, states may prevent federal officials from enforcing laws declared by the state to be unconstitutional, and some federal gun restrictions are in fact unconstitutional — either because they violate the Second Amendment (says Missouri) or are outside the scope of the federal government’s power to regulate commerce (says Montana).

On the first point, the nullifiers are correct: in a 1997 decision, Printz v. United States, the Supreme Court held that “the Federal Government may not compel the States to enact or administer a federal regulatory program.” That case involved the Brady Act of 1993, which established a national system for background checks and commanded state law enforcement officials to conduct them.

Of course, background checks are still required in every state. That’s because federal officials are authorized to enforce their own laws, even if they cannot compel the states to do so. Thus, on the second point, the nullifiers are wrong: states cannot impede federal enforcement of a federal law merely because the state deems it unconstitutional. That is up to the federal courts.

Yes, state legislatures or governors can assert that a federal law offends the Constitution. But as James Madison wrote in his Report of 1800, such declarations are “expressions of opinion” for “exciting reflection. The expositions of the judiciary, on the other hand, are carried into immediate effect.” In assessing constitutionality, our system of governance recognizes one Supreme Court, not 50 individual states.