“Sanctuary city” sounds like a moral label. In court, it is usually something much more technical: a fight over who has to do the work.
Immigration law and removal decisions are federal power. But the day-to-day machinery of policing and jail operations is mostly state and local. That tension is what makes these cases so recurring and so predictable. A state can refuse to lend its officers, jails, or databases to federal immigration enforcement. The federal government can still enforce federal law, but it generally has to do it with federal people and federal resources.
That is why courts keep coming back to the same constitutional architecture: the Tenth Amendment on one side, and the Supremacy Clause on the other.
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The question in sanctuary
When people ask, “Are sanctuary cities constitutional?” they often mean one of two different things. It also helps to say out loud that “sanctuary city” has no single legal definition. It is a catchall for a mix of policies about detainers, jail access, status questions, and information sharing.
Can a city or state refuse to help federal immigration agents?
Can a city or state block federal immigration enforcement?
Those are not the same. Modern doctrine generally protects declining to assist, but it does not protect actively interfering. In real cases, the boundary can be fact-specific, and governments fight about where the line is.
The first question usually turns on the anti-commandeering doctrine . The second can trigger federal preemption, where valid federal law overrides conflicting state or local policy.
Anti-commandeering
The Tenth Amendment reserves to the states (and the people) powers not given to the federal government. On paper, that sounds abstract. In practice, it has produced a concrete rule:
The federal government cannot “commandeer” state and local officials to carry out federal regulatory programs.
This is not about whether federal law is supreme. It is. The Supremacy Clause makes valid federal law the “supreme Law of the Land.” The anti-commandeering doctrine is about who the federal government can order around.
Federal agents can enforce federal law. Congress can fund federal enforcement. Congress can regulate private actors directly. But Congress generally cannot say, “State officers, you must use your time, budgets, jails, databases, and personnel to do our job.”
The Supreme Court cases behind it
New York v. United States (1992): Congress cannot force states to enact or administer a federal regulatory program.
Printz v. United States (1997): Congress cannot require state and local executive officers to implement federal law, even temporarily.
Murphy v. NCAA (2018): Congress cannot use federal law to dictate what state legislatures may or may not authorize, because that still amounts to commandeering.
Put those together and the baseline becomes simple: a state can say no to being drafted into federal enforcement.
Immigration is federal
Immigration is a federal domain in a way that few issues are. The federal government controls admission, removal, visas, and naturalization. States cannot invent their own immigration system, even if state and local officers sometimes interact with immigration-related questions through criminal law, jail booking, or voluntary federal partnerships.
But a “sanctuary” policy is often not a state trying to run immigration. It is a state deciding how its own officers will spend their time, and what level of legal process it will require before using state resources to help with a federal objective.
Common examples include:
Declining to honor certain ICE detainers unless accompanied by a judicial warrant or other adequate legal process.
Limiting when local jails will hold someone past a release time for federal pickup.
Restricting when local officers may ask about immigration status during routine policing.
Limiting access to nonpublic local databases absent a subpoena, warrant, or court order.
The constitutional hinge is usually whether the policy is framed as non-cooperation (often allowed) versus obstruction or a direct conflict with federal requirements (often not).
One related wrinkle is the federal 287(g) program, which lets state and local agencies voluntarily enter agreements to perform certain immigration functions under federal supervision. That is cooperation by choice, not commandeering by force.
The line courts draw
Two policies can look similar in politics and very different in court.
Typical non-cooperation: “Our jail will not extend custody based on a detainer request alone; we will release at the normal time unless there is a judicial warrant or court order.” This is the jurisdiction setting rules for its own custody decisions and legal risk.
Likely legal trouble: “Our officers must physically prevent federal agents from executing a valid federal warrant or administrative arrest authority in our facility,” or “The city will punish private parties for voluntarily providing information to federal immigration authorities.” Those move from allocating local resources to interfering with federal operations or regulating private conduct in a way that can collide with federal law.
Key sanctuary cases
These fights recur in two main forms: (1) state “non-cooperation” statutes that limit what local agencies will do, and (2) federal attempts to use grant conditions to induce cooperation.
United States v. California (the federal challenge to California’s SB 54 and related provisions) is a useful example of how courts separate non-cooperation from interference. At a high level, the Ninth Circuit largely allowed California to limit how its own officers and agencies assist federal immigration enforcement, while finding a narrower restriction on federal access to certain detention-facility areas (an “inspection” or access limitation) preempted. The lesson is not that “SB 54” rose or fell as a single unit, but that courts often evaluate sanctuary provisions one by one.
City of Chicago and City of Philadelphia are the other headline cases readers often encounter because they put the fight into its most common modern form: grant conditions . These cases arose from the Sessions-era effort to add immigration-cooperation requirements to Byrne JAG public-safety grants. Later stages were captioned against subsequent attorneys general, but the core question stayed the same: can the executive branch impose those conditions without clear congressional authorization, and how far can the conditions go before they become unlawful or unconstitutional pressure?
Together, these cases reinforce the working rule readers actually need: the federal government can enforce immigration law itself, and it can sometimes condition spending when Congress clearly says so, but it generally cannot turn state and local agencies into a mandatory workforce for federal enforcement.
Where sanctuary loses
The federal government cannot force state officers to enforce federal immigration law. But under the Supremacy Clause, states also cannot enact policies that conflict with valid federal law or that function as an obstacle to federal objectives.
This is where preemption matters, and it comes in a few familiar forms:
Conflict preemption: A state rule is invalid if complying with both state and federal law is impossible, or if the state rule stands as an obstacle to federal objectives.
Field preemption: Federal regulation is so comprehensive that it leaves no room for states.
Express preemption: Congress says directly that state law is displaced.
In immigration, preemption is real, but it is not unlimited. Courts often distinguish between states trying to create their own immigration penalties and states setting internal rules for state personnel.
The Supreme Court’s major modern marker is Arizona v. United States (2012), which struck down key parts of Arizona’s law because the state tried to operate in a field dominated by federal authority and because certain provisions conflicted with federal priorities.
But Arizona does not itself give the federal government power to force state cooperation. It is mainly about states trying to add an immigration enforcement regime, not states choosing to withhold state help.
What “obstruction” can look like in practice: rules that attempt to bar federal agents from carrying out federal duties inside local facilities when they have lawful authority, or local penalties aimed at third parties for federal cooperation, can move beyond non-cooperation into direct conflict territory.
Information sharing
Any sanctuary explainer that skips 8 U.S.C. § 1373 is leaving out a major flashpoint.
Section 1373 is a federal statute aimed at preventing state and local governments from prohibiting their officials from sharing certain immigration status information with federal authorities. Its litigation history is complicated and has been circuit-dependent, and courts have debated how it fits with anti-commandeering and federalism limits.
The takeaway for readers is practical: there can be a difference between (1) the federal government ordering local officials to do affirmative work for ICE (often unconstitutional commandeering) and (2) the federal government restricting a state’s ability to gag its own employees from voluntarily sharing specific categories of information (a narrower question that has produced contested results and is highly sensitive to statutory text and the exact policy being challenged).
Several post-Murphy decisions have treated § 1373 as constitutionally problematic under anti-commandeering principles, and Byrne JAG litigation often rejected § 1373 as a clean grant-condition hook. The Supreme Court has not produced a single, definitive modern ruling that resolves every § 1373 scenario nationwide, which is why careful drafting and case-specific analysis still matter.
That is why many modern “sanctuary” policies are written to focus on custody decisions, access to facilities, or requiring legal process for nonpublic records, rather than trying to ban every form of communication categorically.
Detainers and the Fourth
A major reason some jurisdictions resist detainers has less to do with immigration politics and more to do with liability.
An ICE detainer is often a request to hold someone past the time they would otherwise be released, so federal agents can take custody. Holding a person longer is a seizure. Seizures trigger the Fourth Amendment .
Many jurisdictions treat detainers as requests, not commands, and some detainers are accompanied by administrative paperwork, including administrative warrants. Courts have differed on what process and proof are sufficient in particular settings, and the details matter.
Still, the risk is straightforward: if a local jail holds someone without a proper legal basis, it risks being sued for unlawful detention. Detainer litigation commonly turns on probable cause and whether the documentation at hand (detainer form, administrative warrant, or other records) supplies a constitutionally adequate basis to extend custody past the normal release time.
This is why many sanctuary policies show up as “warrant requirements” or “release-time rules” rather than as grand statements of defiance.
Funding pressure
Sometimes the federal government tries a different route: not “you must help,” but “if you do not help, you lose money.”
Congress does have broad power to spend and to attach conditions to federal grants. But there are limits. The classic framework comes from South Dakota v. Dole (1987), which is commonly summarized as requiring that conditions be stated clearly, relate to the federal interest in the program, and not violate other constitutional provisions, among other constraints.
There is also a coercion backstop. The Supreme Court has said conditions cannot be so coercive that they become a financial gun to the head. That is the lesson of NFIB v. Sebelius (2012) in the Medicaid expansion context.
Finally, attempted executive-branch efforts to withhold unrelated grants over sanctuary policies have faced serious pushback in the courts, particularly when the condition is not clearly authorized by Congress or is not tied to the purpose of the funding stream.
So a funding condition related to immigration cooperation can be constitutional or unconstitutional depending on who imposed it, how it is written, how directly it relates to the program being funded, and how coercive it is in practice.
What sanctuary is not
It does not mean federal law stops at the city limits.
ICE can still investigate, arrest, detain, and remove people under federal authority. Federal officers can still carry out arrests under federal authority, including through judicial warrants in some contexts and administrative immigration warrants in others. Federal prosecutors can still bring federal charges.
States generally cannot interfere with those federal operations. At the same time, states can often set rules for access to state facilities, state personnel, and nonpublic state records, unless a specific federal requirement validly preempts those limits.
The real question is whether the federal government can require the local police department, the county jail, or the state database administrator to become an extension of ICE.
Under modern Supreme Court doctrine, the answer is usually no when the demand is affirmative enforcement work, but the analysis can change when the dispute is framed as information sharing under federal statute or a direct conflict with a specific federal command.
Bottom line
Sanctuary policies are generally constitutional when they are policies of non-cooperation. That is the anti-commandeering doctrine doing its job: preserving state control over state personnel and budgets. Outcomes can still be provision-specific, and courts often uphold some parts of a policy while rejecting others.
They become legally vulnerable when they cross into obstruction or conflict with federal law. That is where preemption, and sometimes narrower federal statutes about information sharing, can take over.
Quick FAQ
Can the federal government force states to help ICE?
Generally, Congress cannot require state and local officials to administer or enforce federal regulatory programs, including immigration enforcement. That is the Tenth Amendment anti-commandeering rule. Specific disputes can turn on whether the federal demand is truly affirmative enforcement work or something narrower, like an information-sharing restriction under a federal statute.
Can a state ban its police from cooperating with ICE?
A state can usually set rules for its own officers, including limiting when they may assist, when they may hold people in custody, and when they may share nonpublic information. Broad, categorical bans on communication can run into separate issues, including 8 U.S.C. § 1373 litigation and any direct conflicts with valid federal requirements.
Does the Supremacy Clause override sanctuary policies?
Only when there is a genuine conflict with valid federal law. Supremacy does not, by itself, let the federal government draft state officers into federal service.
Is refusing to honor ICE detainers constitutional?
Often yes, because detainers are commonly treated as requests and because local custody extensions raise Fourth Amendment concerns. The legal risk analysis can depend on whether the detainer is supported by adequate probable cause and what legal process accompanies it.