The Limits of Nullification - NYTimes.com 11, anti-gun-control legislators in the Missouri General Assembly are likely to pass a bill, over the governor's veto, that renders almost all federal gun laws void in the state, and even makes it a crime for federal agents to enforce them. Missouri is only the latest state to push back against federal gun laws. In Montana, the Firearms Freedom Act, passed in 2009, purports to exempt any gun manufactured and kept within the state from federal regulations; despite a federal appellate court decision last month invalidating the statute, it has served as a model for new or pending laws in more than a half-dozen states. 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. Strangely, if nullification proponents had their way, Chicago's gun ban, which the Supreme Court invalidated in 2010, might still be in effect. Moreover, if the court had not held in 1960 that nullification "is illegal defiance of constitutional authority," many public schools might have remained segregated. That brings us to the third point: whether the Constitution holds that federal gun laws are unconstitutional. If it does, then states would be justified in preventing enforcement. But despite pleas from the gun rights community, the Supreme Court has not gone that far. Indeed, when the Court overturned Washington's handgun ban in its 2008 decision in District of Columbia v. Heller - in which I was co-counsel to the plaintiff, Dick Heller - Justice Antonin Scalia, who wrote the majority opinion, was careful to note that the right to keep and bear arms is not absolute. He wrote that his opinion did not "cast doubt on longstanding prohibitions on the possession of firearms by felons and the mentally ill, or laws forbidding the carrying of firearms in sensitive places such as schools and government buildings, or laws imposing conditions and qualifications" - like background checks - "on the commercial sale of arms." Subsequent cases will determine which regulations are allowable. But until the courts say otherwise, federal gun laws are presumptively consistent with Second Amendment rights. What about Montana's argument that federal restrictions on guns made and transported entirely within the state exceed Congress's power to regulate interstate commerce? Over protests from some libertarian activists, myself included, the Supreme Court has consistently expanded the federal government's power to regulate commerce to cover any economic act that, in the aggregate, could have a substantial effect on interstate commerce - even if the act is not strictly commercial and is wholly within one state. Meanwhile, nullification battles extend beyond gun laws. Nearly two dozen states have condoned medical marijuana use in defiance of federal restrictions. Washington and Colorado have even legalized recreational marijuana use. At least 23 states have considered bills that nullify the Affordable Care Act of 2010. I fully support those who see risks in the expansion of federal power, particularly when it comes to intrusions on basic rights like gun ownership. However, to defend those rights, we can't begin by flouting the very document that inspires that fight in the first place: the Constitution.