What is the Supremacy Clause?

The Supremacy Clause -- Article VI, Clause 2 of the U.S. Constitution -- reads in full: '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.' Four structural features of this text define the federal hierarchy that flows from it. First, the clause identifies three categories of supreme law: the Constitution itself; federal statutes enacted 'in Pursuance thereof' -- that is, statutes enacted pursuant to a power the Constitution actually grants to Congress, not statutes purporting to exercise authority the Constitution does not authorize; and treaties made under the authority of the United States. A federal statute that exceeds Congress's constitutional authority is not made 'in Pursuance' of the Constitution and is not entitled to Supremacy Clause supremacy: an unconstitutional statute is no law at all, a principle the Court established in Marbury v. Madison, 5 U.S. (1 Cranch) 137 (1803). Second, the clause directs state judges specifically -- not only federal courts -- to be bound by federal supreme law. This was a deliberate departure from the Articles of Confederation, which relied on state courts to enforce national obligations and found that states routinely failed to do so: congressional resolutions were ignored, treaty obligations were dishonored, and no federal mechanism existed to compel compliance. The Convention responded by placing the obligation directly on state judges as a constitutional duty, bypassing state legislatures and making federal supremacy self-enforcing through the judicial branch at both levels of the federal system. Third, the phrase 'any Thing in the Constitution or Laws of any State to the Contrary notwithstanding' forecloses the defense that a state court is bound by its own state constitution or its own state statute to apply state law in conflict with federal law. State constitutional provisions yield to federal law just as state statutes do: no state may insulate a conflict with federal law by embedding it in its own constitution. Fourth, the clause operates on a condition: a federal law must be 'in Pursuance' of the Constitution, and a treaty must be 'made under the Authority of the United States.' The supremacy hierarchy does not make any assertion of federal authority valid; it presupposes that the federal law or treaty being asserted is itself constitutionally valid, leaving to judicial review the threshold determination of whether the federal law exceeds the powers granted to Congress. The structure thus integrates the Supremacy Clause with every other provision granting and limiting federal authority: the clause is not an independent grant of power to Congress but the constitutional mechanism by which valid exercises of federal power override inconsistent state law.

McCulloch v. Maryland, 17 U.S. (4 Wheat.) 316 (1819), applied the Supremacy Clause to establish the doctrine of intergovernmental tax immunity, one of the two central holdings of that foundational case. Chief Justice Marshall, writing for a unanimous Court, held on the first question that Congress had constitutional authority under the Necessary and Proper Clause to charter the Second Bank of the United States. On the second question -- whether the State of Maryland could tax the Bank's Baltimore branch -- Marshall held that the Supremacy Clause barred Maryland's tax and struck it as unconstitutional. Maryland had imposed a stamp tax on all bank notes issued by any bank operating in the state without a Maryland charter, effectively targeting the Second Bank. The state argued that the power to tax is an ordinary incident of state sovereignty and that taxing a federal instrumentality was no different from taxing any other entity doing business within Maryland's borders. Marshall rejected this argument with a structural inference: 'the power to tax involves the power to destroy.' The argument was that if Maryland could tax the Second Bank at any rate it chose, it could levy a rate so high as to prevent the Bank's operation entirely and thereby defeat the federal purpose the Bank served. Because the Supremacy Clause makes the Bank -- a valid federal instrument exercising delegated federal authority -- supreme law, any state action that could negate the Bank's operation is an action that conflicts with federal law and must yield. Marshall acknowledged that the states retained concurrent taxing authority over entities operating within their borders, but held that this taxing power could not be directed at federal instruments without threatening the supremacy of the federal constitutional structure. The holding established what has since been called the doctrine of intergovernmental tax immunity: the federal government and its agents are immune from state taxation when the incidence of the tax falls directly on a federal instrument and the practical effect is to burden the exercise of federal power. The doctrine has been substantially refined since McCulloch: subsequent decisions have held that the immunity does not extend to employees of federal contractors or to nondiscriminatory taxes on income derived from federal employment, and Congress has by statute waived various aspects of federal immunity in contexts where it determined that uniformity with state taxation was appropriate. But the core principle -- that a state may not use its taxing power to destroy a federal instrument or obstruct the execution of federal law -- rests directly on the Supremacy Clause and has never been repudiated.

The doctrine of federal preemption is the principal operational application of the Supremacy Clause in modern constitutional law. The Supremacy Clause's directive that federal law is the supreme Law of the Land and that state law to the contrary yields applies not only to direct conflicts where a state statute literally forbids what federal law requires, but across a spectrum of federal-state interactions that courts have organized into three recognized preemption categories. Express preemption occurs when Congress explicitly states in a federal statute that it intends to displace state law in the area the statute addresses. Congress may expressly preempt all state law in a field (as the Federal Aviation Administration Authorization Act preempts state laws related to rates, routes, and services of air carriers), a defined category of state law (as the Employee Retirement Income Security Act expressly preempts state laws that 'relate to' employee benefit plans covered by ERISA), or only state laws that conflict with specific federal requirements. When express preemption language is present, the central interpretive task is determining the scope of the congressional statement -- which state laws are covered by the preemptive language -- which courts resolve by applying ordinary statutory interpretation canons. Field preemption occurs without any express preemption clause when the scheme of federal regulation is so pervasive and comprehensive that it is reasonable to infer that Congress left no room for the states to supplement it. The touchstone of field preemption is whether Congress, by the comprehensiveness of the federal regulatory structure, manifested an intent to occupy the regulatory domain so thoroughly that state law -- even if not in direct conflict with any specific federal provision -- is displaced because its presence in the field undermines the uniformity and exclusivity of the federal regulatory approach. Field preemption is most commonly found in areas where the federal regulatory scheme is both pervasive and where the federal interest in uniformity is strongest, such as alien registration, nuclear safety regulation, and certain aspects of labor relations. Conflict preemption in its two recognized forms displaces state law that does not conflict expressly and that does not arise in a fully preempted field. Impossibility conflict preemption applies when compliance with both state and federal law simultaneously is physically impossible: if a state law requires conduct that federal law prohibits, or prohibits conduct that federal law requires, no actor can obey both simultaneously, and the state law must yield under the Supremacy Clause. Obstacle conflict preemption applies when compliance with both laws is technically possible but the state law stands as an obstacle to the accomplishment and execution of the full purposes and objectives of Congress in enacting the federal statute. In applying obstacle preemption, courts examine the legislative history and statutory structure of the federal law to determine what purposes Congress intended to advance and whether the state law's operation would frustrate those purposes even if literal compliance with both is not impossible.

Arizona v. United States, 567 U.S. 387 (2012), and Crosby v. National Foreign Trade Council, 530 U.S. 363 (2000), are the Supreme Court's most significant recent applications of the preemption framework in immigration and foreign affairs respectively. Arizona v. United States arose from Arizona's S.B. 1070, enacted in 2010, which created new state criminal offenses and state arrest powers in the immigration context: Section 3 made it a state crime to violate federal alien registration requirements; Section 5(C) made it a state crime for an unauthorized immigrant to apply for, solicit, or perform work; Section 6 authorized state officers to arrest without a warrant any person whom an officer had probable cause to believe had committed a deportable offense; and Section 2(B) required state officers during any lawful stop to make a reasonable attempt to determine the person's immigration status if there was reasonable suspicion the person was unlawfully present. Justice Kennedy, writing for the Court, held that three of the four challenged provisions were preempted by federal immigration law. Section 3 was preempted by field preemption: federal law occupied the field of alien registration through a comprehensive statutory scheme enacted pursuant to congressional authority over immigration under Article I, and Arizona's state criminal enforcement of those same requirements intruded impermissibly into the federal field regardless of whether the state's substantive requirements overlapped with the federal requirements. Section 5(C) was preempted as an obstacle to the federal framework for employer sanctions in immigration: Congress enacted the Immigration Reform and Control Act of 1986 after considering whether to impose criminal penalties on unauthorized workers seeking employment and deliberately chose not to -- Arizona's criminal employment penalty stood as an obstacle to that legislative choice. Section 6 was preempted as an obstacle to federal enforcement discretion: federal law confined warrantless civil immigration arrests to circumstances where an officer had personal knowledge of facts establishing deportability, and Arizona's broader state arrest authority conflicted with the federal enforcement priorities and mechanisms that Congress and the executive branch had established. Section 2(B) was upheld on its face, with the Court noting the provision might be subject to as-applied challenge if its operation caused constitutional violations. Crosby v. National Foreign Trade Council, 530 U.S. 363 (2000), decided unanimously with Justice Souter writing, addressed Massachusetts's act of 1996 barring state agencies from purchasing goods or services from companies doing business with Burma (Myanmar) after the military government there engaged in widespread human rights abuses. Congress subsequently enacted its own Burma sanctions law, giving the President authority to impose targeted sanctions and establishing a calibrated approach that included consultation requirements and mechanisms for the President to adjust the response depending on conditions in Burma and developments in international relations. The Court held that the Massachusetts law was preempted as an obstacle to the federal framework even though the state law served the same general anti-Burma purpose as the federal legislation. Congress's chosen mechanism was deliberate: it provided targeted tools, conditioned their use on specified findings, and contemplated flexibility for the President to adjust the response over time. The Massachusetts blanket prohibition on purchasing from any Burma-connected company conflicted with this deliberate choice by making the state's policy more rigid than Congress had authorized and by undermining the federal government's ability to speak with a single voice on a matter of foreign commercial relations -- an area where uniformity and presidential flexibility are particularly important to the national interest.

For the 2028 presidential election, the Supremacy Clause will shape the new administration's governing capacity in several substantive domains that are certain to be contested in the campaign and during the presidential term beginning January 20, 2029. First, climate and environmental preemption: the Clean Air Act authorizes the Environmental Protection Agency to set national air quality standards, and the Act expressly preserves California's historic authority to set its own stricter standards while preempting other states from doing so without federal waiver. If a new administration weakens federal standards, states seeking to impose stricter requirements face a preemption question depending on whether the federal standards are a floor permitting stricter state law or a ceiling preempting it. Major questions doctrine cases like West Virginia v. EPA, 597 U.S. 697 (2022), intersect with preemption when courts must determine whether an EPA regulation is validly authorized -- an invalid regulation would not receive Supremacy Clause supremacy, leaving state law standing. Second, immigration enforcement: Arizona v. United States establishes the framework but leaves open the extent to which states may assist or supplement federal enforcement, as distinct from independently punishing conduct that federal law addresses differently or leaves to federal discretion. The incoming administration's immigration enforcement priorities -- and whether it deploys federal power to preempt state sanctuary policies or to supplement state enforcement -- will be fought out under the Supremacy Clause in the federal courts. Third, healthcare: the Affordable Care Act's express preemption provisions, ERISA preemption of state benefit-plan regulation, and Medicaid's cooperative-federalism structure all create preemption contests between state insurance regulations and federal requirements. Fourth, election administration: the Elections Clause of Article I, Section 4 gives Congress authority to regulate federal election procedures, and federal statutes on voter registration and election procedures preempt inconsistent state requirements for federal elections; new federal legislation on election security or voter registration would displace conflicting state requirements by force of the Supremacy Clause. Fifth, the Supremacy Clause will also interact with the Tenth Amendment's anti-commandeering doctrine: while the Supremacy Clause preempts state laws that conflict with valid federal law, New York v. United States, 505 U.S. 144 (1992), and Printz v. United States, 521 U.S. 898 (1997), established that the federal government cannot commandeer state legislatures or executive officers to administer federal programs -- a structural limit that narrows how the incoming administration may use the Supremacy Clause to conscript state resources for federal purposes. The 33 Class II Senate seats on the 2028 ballot will determine which party controls the Senate, what legislation reaches the President's desk for signature, and which federal judges are confirmed to district and circuit courts -- the judges who will apply the preemption framework and the Supremacy Clause to the next generation of federal-state conflicts.