The Constitution
Supremacy Clause
Article VI's rule that the Constitution and valid federal law are 'the supreme Law of the Land', overriding conflicting state law.
In full
When valid federal law and state law conflict, federal law wins and state judges are bound to follow it, that is preemption. The clause only operates when the federal law is itself constitutional, and Congress can choose to leave room for states; whether it did is a recurring question in litigation.
Also written as: preemption
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A plain-language explanation for civic education, not legal advice. When in doubt, the official source named above controls.