"Sanctuary city" is not a legal term. There is no federal statute that defines it, no administrative category that cities opt into, and no single policy that all so-called sanctuary jurisdictions share. What the term describes, loosely, is a local government policy that limits cooperation between local law enforcement and federal immigration enforcement agencies — most commonly U.S. Immigration and Customs Enforcement.
Understanding what those policies actually do requires getting past the rhetoric on both sides of an argument that has generated more heat than light.
What Sanctuary Policies Actually Say
Most sanctuary policies take one of three forms. The first, and most common, limits the circumstances under which local police will honor ICE detainer requests — federal requests that a local jail hold an individual beyond their scheduled release date so that ICE agents can take custody. The second limits whether local officers will ask about immigration status during routine police contact. The third restricts the sharing of certain information — like a person's home address from jail records — with federal immigration agencies.
What sanctuary policies generally do not do: prohibit local officers from calling ICE when they encounter someone they believe has committed a crime, prevent ICE from operating within a jurisdiction, or shield anyone from federal immigration enforcement. ICE can and does make arrests in sanctuary cities. The policies limit cooperation, not federal authority.
The Constitutional Argument
The legal foundation for sanctuary policies rests on the anti-commandeering doctrine — a line of Supreme Court precedents holding that the federal government cannot compel state or local governments to implement or enforce federal law. The doctrine was established most clearly in Printz v. United States (1997), which struck down a provision of the Brady Handgun Violence Prevention Act that required local law enforcement to conduct background checks.
Courts have consistently found that ICE detainer requests are voluntary. Local governments are not legally required to honor them, and several federal courts have held that detaining individuals solely on the basis of an ICE detainer — without judicial authorization — may itself constitute a Fourth Amendment violation.
"The Constitution doesn't authorize the federal government to conscript local police departments into immigration enforcement. That's not a progressive legal theory — it's the same logic that courts have applied to gun background checks and drug enforcement for thirty years."
The Defunding Battles
Since 2017, successive administrations have attempted to use federal grant funding as leverage to compel local immigration cooperation. The primary vehicle has been conditions attached to Justice Department grants — requiring certification of cooperation with ICE as a condition of receiving public safety funding.
Those conditions have faced a mixed record in federal court. Several circuit courts have found specific grant conditions unconstitutional as applied, while others have allowed different formulations to proceed. The Supreme Court has not yet resolved the circuit split directly, though cases involving related questions of conditional spending and commandeering have signaled that the Court's current majority may be receptive to broader federal authority arguments than previous courts were.
The Public Safety Debate
Supporters of sanctuary policies argue that limiting local immigration enforcement makes communities safer — not by protecting anyone from consequences for crimes, but by encouraging immigrant residents to report crimes and cooperate with police without fear that doing so will trigger immigration consequences. Several studies have examined crime rates in sanctuary versus non-sanctuary jurisdictions, with mixed results; the most rigorous analyses have found no consistent evidence that sanctuary policies increase crime rates.
Critics contend that honoring ICE detainers for individuals with criminal records is a straightforward public safety measure that should not be complicated by broader immigration debates. Some point to specific cases where individuals released by local authorities went on to commit serious crimes as evidence that cooperation policies matter.
Both arguments carry some evidential weight. The honest accounting is that the research does not definitively resolve the public safety question in either direction, and that individual cases — in both directions — are genuinely tragic without being representative of systemic patterns.
Where Things Stand
As of mid-2026, more than 500 jurisdictions maintain some form of policy limiting local immigration cooperation. Legal challenges continue on multiple fronts. The practical reality is that ICE continues to operate in sanctuary cities — sometimes with local cooperation, sometimes without — and that the policies' effects are more limited, in both directions, than the political debate suggests.