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 the 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 principle that forbids direct commandeering also counts 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."