What legal challenges have U.S. cities faced when passing separation or 'sanctuary' ordinances since 2003?

1. Federal litigation and threats to withhold funds: high-profile suits and executive lists

Cities have been sued by the Department of Justice and targeted by executive orders labeling jurisdictions “sanctuary,” with the government threatening litigation and funding consequences; the DOJ publicly listed jurisdictions and signaled continued litigation to compel compliance with federal immigration enforcement ^{[1] [4] [5]}.

2. Anti‑commandeering doctrine as the principal legal defense and courtroom victories

Localities defending sanctuary rules have repeatedly relied on the anti‑commandeering principle—that the federal government cannot force states or cities to use their resources to enforce federal law—and several courts have accepted that defense, finding federal attempts to condition funding or mandate local enforcement unconstitutional when they effectively commandeer state and local officers ^{[6] [3] [2]}.

3. Preemption fights: when federal law is invoked to nullify local ordinances

The federal government has argued that some local laws are preempted because they obstruct federal immigration duties, prompting suits like United States v. Illinois and other preemption challenges; courts adjudicating these claims have split based on whether local practices pose an actual obstacle to federal enforcement or merely decline to assist it ^{[2] [7]}.

4. State laws and legislative counterattacks — the Texas model and beyond

Several states have moved to ban sanctuary ordinances outright or to punish noncooperation—Texas S.B. 4 is a leading example that spurred cities and counties to sue on home‑rule and constitutional grounds—creating a layer of intrastate conflict where municipal autonomy collides with state law ^{[8] [7]}.

5. Detainer liability and Fourth Amendment constraints driving municipal policy

Legal exposure for honoring federal “detainer” requests without judicial warrants—courts finding potential Fourth Amendment violations and civil liability for unlawful detention—has been a practical driver of sanctuary policies and a basis for some courts upholding local noncooperation as a tool to avoid municipal liability ^{[9] [3]}.

6. Political pressure, congressional proposals, and the theater of hearings

Sanctuary ordinances have become political lightning rods: congressional hearings featuring mayors and aggressive public rhetoric aim to delegitimize local policies and push for federal or state penalties, while legislative proposals in Congress have sought to condition or cut off federal grants to sanctuary jurisdictions—measures that courts have often scrutinized for constitutionality ^{[5] [10]}.

7. Fragmented doctrine and practical ambiguity: no single legal definition, mixed precedents

The term “sanctuary” lacks a uniform legal definition, producing a patchwork of ordinances that differ in scope and vulnerability; courts have issued mixed rulings depending on statute text, the precise local act (detainer refusal, data‑sharing limits, training), and which constitutional principle is foregrounded, leaving unresolved questions about the outer bounds of permissible local noncooperation ^{[9] [11] [12]}.

8. What has proven durable — and what remains unsettled

What has endured is the legal potency of anti‑commandeering and Fourth Amendment limits on detainers, which have saved many local policies from being invalidated outright; what remains unsettled are aggressive federal efforts to redefine preemption and to employ funding levers and criminal theories against local officials—avenues currently contested in courts and likely to produce further splits in precedent ^{[3] [2] [1]}.