Under the Constitution, state and local governments have every right to refuse to help enforce federal law. In cases like Printz v. United States (1997) and New York v. United States (1992), the Supreme Court has ruled that the Tenth Amendment forbids federal “commandeering” of state governments to help enforce federal law. Most of the support for this anti-commandeering principle came from conservative justices such as the late Antonin Scalia, who wrote the majority opinion in Printz.
The looming fight over sanctuary cities is an example of how federalism and constitutional limitations on federal power can sometimes protect vulnerable minorities – in this case undocumented immigrants. States and localities have a reputation for being enemies of minority rights, while the federal government is seen as their protector. That has often been true historically. But sometimes the situation is reversed – a pattern that has become more common in recent years.
At this point, it is not yet clear how far Trump intends to push his deportation agenda. Election exit polls suggest that mass deportation is not a popular policy, with 70% of the public believing that undocumented migrants working in the US should be offered permanent residency, and only 25% indicating they should be deported. The spectacle of the federal government trying to deport large numbers of people in the face of local resistance is unlikely to make good PR for the Trump administration. Perhaps that will lead them to scale back their ambitions.
Should Trump choose to pursue a policy of mass deportation regardless of the potential downsides, sanctuary cities can refuse to cooperate with it. And they will have the Constitution on their side.
UPDATE: It is worth noting that if Congress were to pass a law stripping sanctuary cities of all their federal funding unless they help facilitate federal deportation efforts, it would be unconstitutional under the Supreme Court’s decision striking down the Obamacare Medicaid expansion in NFIB v. Sebelius (2012), which forbids funding conditions so coercive that they amount to a “gun to the head” of a state or local government. While the exact limits of this principle are debatable, denying a state all federal grants for the purpose of compelling cooperation with federal deportation policy surely qualifies, if anything does. At the very least, that would be true for local governments that depend on federal funds for a substantial proportion of their budget.