What is the Supremacy Clause?

The second paragraph of Article VI of the Constitution reads: "This Constitution, and the Laws of the United States which shall be made in Pursuance thereof; and all Treaties made, or which shall be made, under the Authority of the United States, shall be the supreme Law of the Land; and the Judges in every State shall be bound thereby, any Thing in the Constitution or Laws of any State to the Contrary notwithstanding."¹ The next paragraph requires state and federal officers alike to swear an oath to support the Constitution.¹

The clause does not list what states may or may not do. Courts have built several doctrines on it. Preemption decides when a federal law displaces a state law. Intergovernmental immunity protects the federal government itself from state laws that regulate it directly or discriminate against it.² A related immunity can shield federal officers from state prosecution for acts within their duties. A separate rule runs the other way: the federal government "may not compel the States to implement, by legislation or executive action, federal regulatory programs," in the words of the Supreme Court's 1997 decision in Printz v. United States.³

When a valid federal law and a state law collide, the federal law controls. Congress can say in a statute that it displaces state law. Courts also treat a state law as preempted when it conflicts with federal law, which the Supreme Court has described as cases where "compliance with both federal and state regulations is a physical impossibility" and those where the state law "stands as an obstacle to the accomplishment and execution of the full purposes and objectives of Congress."³

Courts apply the obstacle test with caution. The Ninth Circuit, quoting the Supreme Court, wrote in 2019 that implied preemption analysis does not justify a "freewheeling judicial inquiry into whether a state statute is in tension with federal objectives."³

Intergovernmental immunity dates to McCulloch v. Maryland in 1819, when the Supreme Court struck down Maryland's tax on the Bank of the United States. Maryland imposed no comparable tax on other banks in the state. Chief Justice John Marshall wrote that under the Supremacy Clause, "the States have no power, by taxation or otherwise, to retard, impede, burden, or in any manner control" the operation of federal laws.²

The Supreme Court has since narrowed the rule. It once read the doctrine to bar any state law that raised the federal government's costs. It now bars state laws that "regulat[e] the United States directly or discriminat[e] against the Federal Government or those with whom it deals," such as contractors. A law discriminates if it singles out the federal government or its contractors for less favorable treatment. In United States v. Washington (2022), a unanimous Court used that test to strike down a Washington workers' compensation law that made claims easier only for federal contract workers at the Hanford nuclear site. Congress can waive the immunity, the Court said, but it had not done so for that law.² The Ninth Circuit held in 2019 that there is no exception for small burdens: any economic burden a state imposes on the federal government in a discriminatory way is unlawful.³

Laws that bar police from covering their faces, or require them to show identification, have tested both halves of the rule. In February 2026, a federal district court barred California from enforcing its face-covering ban, the No Secret Police Act, against federal officers because the law exempted California's own state officers. The court found the United States had not shown that the mask law or a companion identification law directly regulated the federal government, and it declined to block the identification law.⁴ In April 2026, the Ninth Circuit took a different view of the identification law. In an order blocking it as to federal officers while the appeal proceeds, the court wrote that the law "attempts to directly regulate the United States in its performance of governmental functions."⁵ On September 29, 2026, California extended its face-covering rules to state officers.⁶

Bridget Lavender, a staff attorney at the University of Wisconsin Law School's State Democracy Research Initiative, wrote in State Court Report that lawmakers in at least 30 other states had introduced or considered similar bills, that most of them cover every level of law enforcement, and that the open question is whether such a ban directly regulates federal officers or only affects them incidentally.⁷ In Connecticut, federal officials and Republican state legislators have cited the Supremacy Clause against a state mask law that the Department of Homeland Security has sued to block. State Senate President Pro Tempore Martin Looney, a Democrat, said lawmakers took care not to subject federal officers to anything the state does not require of its own officers.⁸

The Supremacy Clause makes valid federal law binding in every state, but it does not require states to help enforce it. Under the anticommandeering rule, which courts draw from the Tenth Amendment, Congress cannot order states to carry out federal programs.³

In 2019, the Ninth Circuit applied that rule to SB 54, California's law limiting cooperation between state and local police and federal immigration authorities. The United States argued that the law obstructed immigration enforcement. The court disagreed: "whatever the wisdom of the underlying policy adopted by California, that frustration is permissible, because California has the right, pursuant to the anticommandeering rule, to refrain from assisting with federal efforts." It quoted the district court's reasoning: "refusing to help is not the same as impeding."³ In the same ruling, the court held that a provision of another California law, AB 103, requiring state inspections of immigration detention facilities to examine how detainees were arrested and transferred, discriminated against the federal government.³

States can charge federal officers with state crimes. "The law is clear that there is no absolute immunity for federal officers from state law," Alicia Bannon, editor of the Brennan Center's State Court Report, wrote in January 2026. She said Vice President JD Vance and other administration officials had suggested otherwise.⁹

What federal officers can claim is Supremacy Clause immunity, which traces to In re Neagle (1890). David Neagle, a deputy U.S. marshal assigned to protect Supreme Court Justice Stephen Field, shot and killed David Terry, a former chief justice of California, when Terry attacked Field, and local authorities arrested Neagle. The Supreme Court held him immune from state prosecution because he was carrying out his federal duties and did "no more than what was necessary and proper for him to do."⁹ Under the framework Bannon describes, a state prosecution can go forward if the state shows the officer acted outside federal duties or acted unreasonably or unlawfully, and an officer who claims the immunity can move the case to federal court. In 2001, the Ninth Circuit allowed an Idaho county prosecutor to pursue a manslaughter case against an FBI sniper who had killed an unarmed woman during a raid on a cabin near Ruby Ridge, Idaho.⁹