Yesterday, President Donald Trump signed an executive order denying federal funding to sanctuary cities – jurisdictions that choose not to cooperate with federal efforts to deport undocumented immigrants. The order has serious constitutional problems. Unless interpreted very narrowly, it is both unconstitutional and a very dangerous precedent. Trump and future presidents could use it to seriously undermine constitutional federalism by forcing dissenting cities and states to obey presidential dictates, even without authorization from Congress. The circumvention of Congress makes the order a threat to separation of powers, as well.

The order indicates that sanctuary cities “that fail to comply with applicable Federal law do not receive Federal funds, except as mandated by law.” More specifically, it mandates that “the Attorney General and the [Homeland Security] Secretary, in their discretion and to the extent consistent with law, shall ensure that jurisdictions that willfully refuse to comply with 8 U.S.C. 1373 (sanctuary jurisdictions) are not eligible to receive Federal grants, except as deemed necessary for law enforcement purposes by the Attorney General or the Secretary.”

Section 1373 mandates that

a Federal, State, or local government entity or official may not prohibit, or in any way restrict, any government entity or official from sending to, or receiving from, the Immigration and Naturalization Service information regarding the citizenship or immigration status, lawful or unlawful, of any individual.

There are two serious constitutional problems with conditioning federal grants to sanctuary cities on compliance with Section 1373.

First, longstanding Supreme Court precedent mandates that the federal government may not impose conditions on grants to states and localities unless the conditions are “unambiguously” stated in the text of the law “so that the States can knowingly decide whether or not to accept those funds.”

Few if any federal grants to sanctuary cities are explicitly conditioned on compliance with Section 1373. Any such condition must be passed by Congress, and may only apply to new grants, not ones that have already been appropriated. The executive cannot simply make up new conditions on its own and impose them on state and local governments. Doing so undermines both separation of powers and federalism.

Even aside from Trump’s dubious effort to tie it to federal grants, Section 1373 is itself unconstitutional. The Supreme Court has repeatedly ruled that the federal government may not “commandeer” state and local officials by compelling them to enforce federal law. Such policies violate the Tenth Amendment.

Section 1373 attempts to circumvent this prohibition by forbidding higher-level state and local officials from mandating that lower-level ones refuse to help in enforcing federal policy. But the same principles that forbid direct commandeering also count against Section 1373. As the late conservative Justice Antonin Scalia explained in Printz v. United States, the purpose of the anti-commandeering doctrine is the “[p]reservation of the States as independent and autonomous political entities.”

That independence and autonomy is massively undermined if the federal government can take away the states’ power to decide what state and local officials may do while on the job. As Scalia put it in the same opinion, federal law violates the Tenth Amendment if it “requires [state employees] to provide information that belongs to the State and is available to them only in their official capacity.” The same is true if, as in the case of Section 1373, the federal government tries to prevent states from controlling their employees’ use of information that “is available to them only in their official capacity.”

Some defenders of Trump’s policy claim that the anti-commandeering rule does not apply to federal laws that mandate disclosure of information. I addressed that argument here. Quite simply, there is no information-disclosure exception to the Tenth Amendment, and it would be very dangerous for the courts to create one.

The Trump administration might try to push a more expansive interpretation of Section 1373 that goes beyond information-sharing and extends to actual detention of undocumented immigrants targeted for deportation. That would only make the law more clearly unconstitutional than if it were limited to information. Pushing state officials to detain people is an even greater interference with state “independence and autonomy” than pressuring them to disclose information.

Unlike the question raised by Trump’s attempt to impose grant conditions not authorized by Congress, the anti-commandeering issue raised by Section 1373 has not yet been directly addressed by the Supreme Court (though the law was upheld in a badly flawed lower court decision back in 1999). We cannot be certain what will happen when and if the justices take up this issue. But the principles underlying the Court’s anti-commandeering cases should lead it to strike down this law.

The constitutional issues raised by Trump’s executive order are not mere technicalities. If the president can make up new conditions on federal grants to the states and impose them without specific, advance congressional authorization, he would have a powerful tool for bullying states and localities into submission on a wide range of issues. Such an executive power-grab also undermines separation of powers. Congress, not the president, has the constitutional authority to attach conditions to federal grants to state governments.

Even if the power-grab is limited to withholding funds when states or localities violate other federal laws and regulations, it is still a grave menace. There are literally thousands of federal laws and regulations on the books. No jurisdiction can fully comply with all of them. If the president can withhold funds from any state or locality that violates any federal law, without needing specific authorization from Congress, he would have sweeping authority over state officials.

Trump’s order is exactly the kind of high-handed federal coercion of states and undermining of separation of powers that outraged conservatives under Obama. In fact, Obama did not go as far as Trump seems to do here. Obama never claimed sweeping authority to impose new conditions on federal grants beyond those specifically imposed by Congress. Even those who sympathize Trump’s objectives in this case should pause to consider whether they want presidents to have this kind of power going forward. Trump’s use of it today could easily serve as a model for a liberal Democratic president tomorrow.

I have my reservations about some aspects of the Supreme Court’s conditional spending precedents. The doctrine is far from ideal. But it is far preferable to letting the president make up conditions and impose them without congressional authorization.

The administration could potentially avoid these constitutional problems if they interpret the order very narrowly. The text states that federal funds will only be withheld from sanctuary cities “to the extent consistent with law.”

Taken literally, that might bar any withholding of funds not explicitly conditioned on compliance with Section 1373. After all, the Constitution is the law, and the Constitution does not allow the president to impose grant conditions not specifically authorized by Congress.

But such a narrow interpretation would make the order largely superfluous. After all, the federal government is already required to withhold funds from jurisdictions that disobey conditions specifically imposed by Congress. Still, it is better that the order be redundant than that it mount an assault on federalism and separation of powers.

If Trump does not withdraw this order or adopt a narrow interpretation of it, sanctuary cities should fight him in court. And all who care about constitutional federalism and separation of powers should support them.

This post first appeared at the Volokh Conspiracy.