← The hardest concepts in United States law
In one breath
Under the Supremacy Clause, valid federal law overrides conflicting state law, but it displaces a state law only in particular ways: by saying so expressly, by occupying a whole field, or by making compliance with both impossible or defeating the federal purpose. Courts begin by asking what Congress meant to displace.
The picture
The franchise and the local branch
A national restaurant chain gives each local branch a manual. Whether a branch may add its own touches depends on the manual. Sometimes the manual says in terms, “branches may not set their own prices.” That is express preemption. Sometimes the manual is so detailed about everything, down to the temperature of the fryers, that nothing is left for the branch to decide: that is field preemption. And sometimes the manual says nothing on a point, but a branch rule would make it impossible to follow the manual, or would defeat what the manual is plainly there to achieve: that is conflict preemption.
Otherwise a branch is free to run itself. A manual that tells branches which uniform to wear does not stop a branch from also adding a seasonal menu. A branch is displaced only to the extent that the manual, read fairly, intends, and the manual is read against the background of what branches have always decided for themselves.
Where the picture stops working
A franchise is a contract that a branch can walk away from. The Supremacy Clause is a rule of constitutional hierarchy that binds the states. Congress can also leave room on purpose, by a savings clause, and courts differ over how to read these in hard cases and over how far the “obstacle” branch of conflict preemption reaches.
How it really works
- 1Find the federal source: a statute, a treaty or a valid regulation. Regulations of a federal agency can preempt if they are within the agency’s delegated authority.
- 2Look for express preemption language and read its scope with care, together with any savings clause that preserves state law.
- 3If there is none, or it is unclear, ask whether Congress occupied the whole field, which may be shown by the pervasiveness of the scheme or by a federal interest so dominant that supplementation by the states is not assumed.
- 4Ask about conflict. Is it impossible to comply with both laws, or does the state law stand as an obstacle to the full purposes and objectives of Congress (Hines v. Davidowitz)?
- 5Apply the presumption. In fields traditionally regulated by the states, courts assume Congress did not intend to displace state law unless that intent is clear (Rice v. Santa Fe Elevator).
Why people get it wrong
- Thinking any overlap means preemption. Federal and state law often coexist, and many statutes expect them to.
- Reasoning that federal law preempts because it is federal. It does so only where Congress meant it to, and only within the area the statute reaches.
- Mixing it up with the dormant Commerce Clause. Preemption requires an actual federal law; the dormant doctrine applies where there is none.
- Overlooking regulations. Agency rules can preempt, not just statutes.
- Assuming a federal standard is a ceiling. Depending on the statute it may be a floor that states are free to exceed.
A worked example
The facts are invented
Congress passes the invented Fenn Ferry Safety Act, which requires every commercial ferry to carry a set number of life jackets and states that no state may impose any requirement for life-saving equipment different from the Act’s. A coastal state requires one extra jacket for every ten passengers. That is a different requirement on exactly the subject the Act names, so it is expressly preempted.
Now suppose the Act and its regulations govern navigation lights in great detail, but say nothing about preemption. A state tries to require an additional flashing light on river ferries. Here a court asks whether the federal scheme is so thorough that it leaves no room (field preemption), and whether the extra light would confuse the uniform national signals the scheme exists to create (conflict). If the Act included a clause preserving state-law injury claims, a passenger’s damages claim under state law would not be preempted by it.
Try it
A federal statute sets a minimum safety standard for ladders and has no clause about state law. A state requires ladders to meet a higher standard. Is the state law preempted?
Landmark authorities
Read the sources themselves; this list is a starting point.
- U.S. Constitution, art. VI, cl. 2 (the Supremacy Clause)
- Hines v. Davidowitz, 312 U.S. 52 (1941)
- Rice v. Santa Fe Elevator Corp., 331 U.S. 218 (1947)
- Florida Lime & Avocado Growers, Inc. v. Paul, 373 U.S. 132 (1963)
Where to go next
Case briefs
More hard concepts in United States law
- Standing, ripeness and mootness (Constitutional law)
- The levels of scrutiny in equal protection (Constitutional law)
- The dormant Commerce Clause (Constitutional law)
- The Eleventh Amendment and state sovereign immunity (Constitutional law)
This page is an educational overview. It is not legal advice and not a substitute for the sources: the statute, the rule, the case itself. United States law is made in fifty states and in the federal system, and the rules differ between them. Each page states the usual formulation of the rule and names landmark decisions; your own state, circuit or course may use different wording. Check the primary sources before relying on any rule.