Yesterday Attorney General Jeff Sessions threatened to withhold, terminate, and "claw-back" federal funding for so-called sanctuary cities and states, which are those jurisdictions that either won't help the federal government round up and deport undocumented immigrants or otherwise refuse to participate in the enforcement of federal immigration laws. "I urge our nation's states and cities to consider carefully the harm they are doing to their citizens by refusing to enforce our immigration laws, and to re-think these policies," Sessions said. "Such policies make their cities and states less safe, and put them at risk of losing valuable federal dollars."

Sessions may not like the idea of sanctuary cities, but sanctuary cites are protected by both the Constitution and by Supreme Court precedent. As Justice Antonin Scalia observed in his 2007 majority opinion in Printz v. United States, "the Federal Government may neither issue directives requiring the States to address particular problems, nor command the States' officers, or those of their political subdivisions, to administer or enforce a federal regulatory program." In other words, thanks to the 10th Amendment and to the constitutional principles of federalism, the federal government may not commandeer the states for federal purposes. What that means here is that state and local officials have every right to refuse to enforce federal immigration laws.

But what about when the federal government threatens to withhold federal funding from those states or cities that refuse to do its bidding? Yes, that too can be unconstitutional. Under Article I, Section 8, Congress possesses to powers to tax and to spend. But as the Supreme Court has repeatedly made clear, those powers may not be used in illegal ways. For example, the Court has said that it is unconstitutional for the feds to impose "coercive" conditions on federal grants to the states.

To be sure, the Supreme Court has allowed the feds to attach certain strings to federal dollars. Most famously, in South Dakota v. Dole (1987), that state lost 5 percent of its federal highway funding because it refused to raise its legal drinking age from 19 to 21. The Supreme Court ruled for the federal government in that dispute because it found the threatened loss of just 5 percent of federal highway dollars to be non-coercive. It was a nudge, not a gun to the head.

But South Dakota v. Dole is not the last word on the matter. The most recent case on point is National Federation of Independent Business v. Sebelius, the 2012 dispute over the constitutionality of the Patient Protection and Affordable Care Act. One of the central questions in that case was whether Congress had overstepped its lawful bounds when it threatened to cut off all existing Medicaid funding to any state that refused to expand Medicaid in accordance with Obamacare.

The Obama administration lost on that question by 7-2. The federal government's Medicaid expansion amounted to a "gun to the head," the Supreme Court held. "A State that opts out of the Affordable Care Act's expansion in health care coverage…stands to lose not merely 'a relatively small percentage' of its existing Medicaid funding, but all of it." That sort of "economic dragooning…leaves the States with no real option but to acquiesce."

Jeff Sessions's threats against sanctuary cities would seem to be equally unconstitutional under this standard. Just like the Medicaid expansion, Sessions aims to dragoon state and local officials and leave them "no real option but to acquiesce." If the Trump administration makes good on those threats, its actions will most likely violate the Constitution.