One of the most important issues state attorney general offices litigate is whether federal law preempts particular state laws and enforcement actions. Under the Constitution’s Supremacy Clause, federal law is the “supreme Law of the Land” and overrides conflicting state law.

Congress sometimes expressly provides that state laws on a given topic are preempted (this is known as “express preemption”). Other times state laws are held preempted by courts because it is impossible to comply with both a federal and state law or because the state law frustrates accomplishment of the federal law’s objectives (this is known as “implied preemption”).

Whether a federal statute should expressly preempt state laws is a complicated policy question that is resolved in the halls of Congress. State attorneys general have often urged Congress not to expressly preempt state laws.

Addressing Preemption in the Courts

Attorneys general most often address preemption in the courts when they defend state laws and enforcement actions against preemption claims. Preemption cases raise a variety of difficult questions:

• What is the scope of an express preemption provision?
• Is it really impossible to comply with both federal and state law?
• What do the federal and state laws substantively mean?
• What were Congress’ objectives in enacting the federal law?
• Does the state law burden the federal objectives and, if so, to what extent?
• Did Congress have any preemptive intent?
• Is the obstacle the state law imposes on federal objectives significant enough to warrant preempting the state law?

In resolving preemption cases and answering these questions, courts begin with a presumption that state laws are not preempted.

Preemption Cases in the U.S. Supreme Court

Preemption cases handled by state attorney general offices are too many to count. Noteworthy recent preemption cases in the United States Supreme Court where attorneys general were parties or amicus curiae addressed:

• Whether the Atomic Energy Act preempts a Virginia law that bans uranium mining in the state. Virginia Uranium, Inc. v. Warren, 587 U.S. ___ (2019).
• Whether Vermont tort law, which imposed a duty on a drug manufacturer to strengthen a drug’s warning label, was preempted because the Food and Drug Administration approved the exact text of the original label. Wyeth v. Levine, 555 U.S. 555 (2009).
• Whether a regulation issued by the Comptroller of the Currency validly preempted New York’s efforts to enforce its fair lending laws against national banks. Cuomo v. Clearing House Association, 557 U.S. 519 (2009).
• Whether an Arizona law imposing sanctions on employers who hire unauthorized aliens was preempted by federal immigration laws. Arizona v. United States, 567 S. 387 (2012).
• Whether the Federal Cigarette Labeling and Advertising Act preempted a lawsuit brought under the Maine Unfair Trade Practices Act against tobacco companies based on their fraudulent advertising that “light” cigarettes delivered less tar and nicotine than regular brands. Altria Group, Inc. v. Good, 555 U.S. 70 (2008).
• Whether an auto manufacturer’s compliance with the 1989 version of Federal Motor Safety Standard 208 ― which gave it the choice of installing simple lap belts or lap-and-shoulder belts on rear inner seats ― preempted a state tort suit that would impose liability upon manufacturers that chose to install a simple lap belt. Williamson v. Mazda Motor of America, Inc., 562 U.S. 323 (2011).

A thorough description of preemption law can be found in Dan Schweitzer, The Law of Preemption (2d ed. 2011) (NAGTRI).