The Supremacy Clause makes valid federal law "the supreme Law of the Land … any Thing in the Constitution or Laws of any State to the Contrary notwithstanding." When federal and state law collide, federal law wins. That is preemption. You saw the basics in Federal and State Law; here are the categories courts use.
| Kind | When it applies | Example |
|---|---|---|
| Express preemption | The federal statute says so in words | "No State may impose labeling requirements different from these." |
| Field preemption | Federal regulation is so pervasive, or the federal interest so dominant, that Congress left no room for states | Alien registration (Arizona v. United States, 2012) |
| Conflict: impossibility | It is impossible to comply with both federal and state law | Federal law requires a label; state law forbids it. |
| Conflict: obstacle | State law stands as an obstacle to the full purposes of federal law | A state penalty scheme that upsets a balance Congress chose |
Why it works
Key idea
Worked example: Arizona v. United States (2012)
- Section 3 (a state crime for failing to carry federal registration papers): field preemption; Congress occupied the field of alien registration.
- Section 5(C) (a state crime for unauthorized workers seeking work): obstacle to Congress's choice to penalize employers, not workers.
- Section 6 (warrantless arrests for removability): obstacle to the federal system for deciding removal.
- Section 2(B) (officers to check immigration status during lawful stops): not preempted on its face, before the state courts had applied it.
Answer: Three of four provisions preempted, 5 to 3 (Justice Kennedy for the Court; Justice Kagan did not take part).
Worked example: Overlap without preemption
- Express: the federal statute says nothing. No.
- Field: minimum standards suggest Congress expected others to add to them. Likely no.
- Impossibility: a maker can meet both by meeting the stricter state rule. No.
- Obstacle: unless the federal purpose was a single national standard, no.
Answer: Probably not preempted. Overlap alone is not preemption.
Common mistake
Preemption is how Congress normally overrides states, and it is lawful as long as the federal law regulates private conduct within an enumerated power. Compare anti-commandeering, which forbids Congress to give orders to state governments. Hamilton discussed federal supremacy in Federalist No. 33.