The Supremacy Clause and Preemption

I've found when working with students that many understand the purpose of the Supremacy Clause of the Constitution. But the MBE might test some very fine details that are worth remembering.

First to note is the idea of express preemption. A federal law may expressly provide that the states may not adopt laws concerning the subject matter of federal legislation. More difficult (and subtle) is implied preemption. If a state law conflicts with a federal law, the state law will be held to be impliedly preempted. Similarly, if a state law prevents the achievement of a federal objective, it will be held to be impliedly preempted. And this is so even if the state law was enacted for a valid purpose other than to prevent the achievement of the federal law.

You'll also want to keep in mind the situations in which a federal statute occupies the field.  In those instances, the statute will preclude any state regulation even if the state regulation does not conflict with a federal law. Determining whether the federal regulation occupies the field will be the objective of a court which will use a variety of factors in determining congressional intent.

In all of the above (and especially when determining whether the federal court has occupied the field) the court will start with the presumption that state powers are not to be superseded unless that was the clear intent of Congress.