Sanctuary cities are cities that have adopted a policy of not prosecuting undocumented immigrants for violating federal immigrations laws. The idea of sanctuary cities dates back thousands of years and was instituted in 1979 in Los Angeles. San Francisco followed suit in 1989.

Timeline

Assessments

Ilya Somin — The Washington Post

Somin examines the case for sanctuary cities from the perspective of the Constitution and Federalism in light of Trump's threats to cut funding:

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.

Trump has said that he intends to break the resistance of sanctuary cities by cutting off all of their federal funding. The cities might continue resisting even if they do lose some federal funds. But Trump’s threat is not as formidable as it might seem.

Few if any federal grants to state and local governments are conditioned on cooperation with federal deportation efforts. The Supreme Court has long ruled that conditions on federal grants to state and local governments are not enforceable unless they are “unambiguously” stated in the text of the law “so that the States can knowingly decide whether or not to accept those funds.”