In a little over two years, four states have legalized the cultivation and distribution of marijuana—Colorado, Washington, Oregon, and Alaska—under state law. Of course, marijuana is still illegal under federal law, but the Obama administration has taken a hands-off approach, explaining in a 2013 Justice Department memo that prosecution of marijuana cases would henceforth be a limited enforcement priority.

Meanwhile, potent cannabis grown with scientific techniques under license has started finding its way into states where the weed is still illegal. Last December, Nebraska and Oklahoma sued Colorado in the Supreme Court, arguing that its regulations undermine the federal Controlled Substances Act, create a nuisance in their states, and should be struck down.  

Normally, conservative states like Nebraska and Oklahoma champion state prerogatives, while progressive ones fight for federal uniformity. But this time the roles are almost perfectly reversed, with some conservative states championing federal uniformity, and progressive ones arguing for state diversity. The controversy has revealed an interesting fissure in the conservative movement, between pro-government “law and order” types, and anti-government “stay out of my life” types. The implications could go far beyond the happy Rastafarian admonition: “Legalize it, don’t criticize it.”

When state and federal laws are inconsistent, the default setting is that both are enforced, by their respective enforcers. State and federal governments are independent sovereigns whose laws operate independently on the citizens subject to them. If you have a Texas driver’s license, you can drive on highways in west Texas and run over all the lizards you want—as far as Texas is concerned. But some of those lizards are endangered species under federal law, and running them over is prohibited—as far as Congress is concerned. You can keep your license, but you may get an unpleasant visit from the U.S. Fish and Wildlife Service.

As long as citizens can comply with both sets of laws and Congress can successfully operate its own laws, there should be no conflict. But where there is a conflict, state law must give way, because the Constitution provides that federal law is the supreme law of the land. For example, when state law “stands as an obstacle to the accomplishment and execution of the full purposes and objectives of Congress,” the Supreme Court has said that state law is preempted.

Does state legalization of marijuana interfere with “the full purposes and objectives of Congress” in its own prohibition of marijuana? It does, but only in a very peculiar way. And here’s where the story gets really interesting.

Federal drug-enforcement agencies like the FBI and DEA focus on “big fish” involved in large-scale drug-trafficking. But federal drug enforcement is built atop the edifice of state law enforcement. “It’s a division of labor,” says Percy Martinez, a criminal-defense attorney and former prosecutor in Miami. “States go after the little fish and the feds go after the big fish. But if you don’t go after the little fish, then sometimes you can’t get the big fish.”

Federal law criminalizes all aspects of the illegal drug trade, big and small, but the federal government can’t enforce all of that on its own, because it simply doesn’t have enough money or manpower. State and local law enforcement agencies are the ones with hundreds of thousands of cops on the streets. The DEA’s entire budget would fund perhaps six months of operations for the NYPD.

Federal drug enforcement, in other words, depends on state agencies for “the accomplishment and execution of the full purposes and objectives of Congress.” But states have no obligation to further the purposes and objectives of Congress. Therefore their refusal to do so—or even their adoption of laws that have contrary purposes and objectives—can’t be preempted by federal law.

The complaint filed by Nebraska and Oklahoma alleges that “In passing and enforcing Amendment 64, the State of Colorado has created a dangerous gap in the federal drug control system enacted by the United States Congress.” That’s clearly wrong. Amendment 64 may have exposed a dangerous gap in the federal drug-control system. But the gap was created by Congress, not Colorado. As the Justice Department memo explains, in most cases “the federal government has traditionally relied on states and local law enforcement agencies to address marijuana activity through enforcement of their own narcotics laws.” Congress in effect chose to depend on the states, by passing laws more sweeping than it can enforce on its own. States can’t be penalized for that under the doctrine of preemption.

The root of this problem goes all the way back to Franklin D. Roosevelt’s New Deal. For the first 150 years of the republic, Congress’s power to regulate commerce “among the several states” meant that Congress couldn’t regulate transactions that were purely internal to one state, which were the exclusive jurisdiction of that state. But, under withering pressure from Roosevelt’s New Deal coalition, the Supreme Court in effect rewrote the Constitution to permit extensive federal regulation of all commercial activity.

State jurisdiction did not recede with the expansion of federal power, however. Henceforth state and federal power over commercial activity would substantially overlap. That is how we got laws criminalizing the same drugs with similar penalties at both the federal and state level. Conservatives never complained that there was double jeopardy in the accumulation of those penalties.

That is also how the federal government started expanding its power—by coopting the regulatory programs of state agencies in schemes known as “cooperate federalism.” It’s this point that the debate over state legalization has missed so far.

In the Obama era, conservatives have become particularly unified on the need for strong constitutional limits to federal power. They have consequently become increasingly strong proponents of “federalism”—the preservation of state prerogatives within America’s scheme of divided government.

Jonathan Adler, a conservative law professor, has described conservatives who argue that strong federal drug laws should displace state legalization as “fair-weather federalists.” Whatever you may think of the wisdom of legalizing marijuana, Adler is right.

In a recent Wall Street Journal op-ed supporting the lawsuit against Colorado, David Rivkin and Elizabeth Price Foley write, “If the [Controlled Substances Act] is a valid federal statute, the U.S. Constitution’s supremacy clause (Article VI, paragraph 2) instructs that conflicting state laws cannot be allowed to stand.” That may be true, but it doesn’t get you very far. Just because a state law is inconsistent with a federal law doesn’t mean there’s a conflict. Conversely, state laws can be entirely consistent with federal law and still violate the Supremacy Clause.

The question is whether state regulation is preempted by federal regulation. In the case of criminal law, it’s clear that states are not preempted. State and federal criminal law have always existed comfortably side by side. And the fact that states undermine a federal scheme by not playing the part Congress wants them to is irrelevant to the preemption analysis. Congress shouldn’t pass laws that it is unwilling or unable to enforce.

Conservatives are right to complain about Obama’s “lawless” elevation of prosecutorial discretion to a kind of extra-constitutional veto in his recent executive order on immigration. But the administration’s decision not to spend scarce resources on the prosecution of marijuana cases is not in that category. Rather, it is a concession to the reality created by Congress’s over-criminalization of matters that are better left to the states.

Rivkin and Foley quote Arizona v. United States (2012), in which the Supreme Court struck down parts of Arizona’s infamous SB 1070, for the proposition that “when the federal government doesn’t enforce its own laws, states still ‘may not pursue policies that undermine federal law.’” But what the Court said in that case was that the federal immigration scheme is so comprehensive that it preempts any state laws touching on immigration—even complementary laws. Nobody would argue that federal criminal law is so comprehensive as to preempt state criminal law.

Rivkin and Foley also repair to Gonzales v. Raich (2005), in which the Court ruled that California’s medical-marijuana laws couldn’t protect citizens from the reach of the federal prohibition on marijuana, because growing and smoking marijuana for personal use has a substantial effect on interstate commerce for purposes of the federal prohibition. But that case did not strike down the California law. It merely confirmed what we’ve known since the New Deal, which is that Congress can regulate virtually everything individuals do under its power to regulate interstate commerce.

Through all its expansions of federal power, the Supreme Court has never held that Congress can dragoon the states into implementing federal law. And that leaves us with one of the most important limitations that still remain on federal power, namely that Congress simply can’t afford to regulate everything under the sun.

Conservatives and progressives alike have a common interest in preserving that limitation. Diversity in state regulation is good for democracy, and depends on the maintenance of some limits on the federal government’s power. Conservatives should stick to that principle even when it cuts against their preferences on social policies that are better left to the states. And progressives should see the wisdom in forcing the federal government to pass laws that it can actually enforce, and let the states mind their own business.