The proper relationship between states and the national government has been one of the most hotly contested debates throughout American history. While the drafters of the Constitution reached several compromises and left some areas intentionally vague, they made clear that federal law should supersede state law through a piece of wording known today as the Supremacy Clause.
The Constitution's Supremacy Clause makes it clear that any law passed by Congress trumps state law or constitution. Article IV, clause 2, specifically reads, "This Constitution, and the Laws of the United States which shall be made in Pursuance thereof ... shall be the supreme Law of the Land; and the Judges in every State shall be bound thereby, any Thing in the Constitution or Laws of any State to the Contrary notwithstanding." But as plainly as this idea is spelled out in the text of the Constitution, application of the principle can be complex.
Though the U.S. government is a separate national entity and not a federation of states, the fact that states remain as sovereign entities within the U.S. system is a unique feature of American government. The principle that states retain more than nominal sovereignty within the constitutional framework is called federalism, and is most frequently associated with the Ninth and 10th Amendments. Out of respect for federalism, the U.S. Supreme Court will not presume preemption of a state law by a federal law unless doing so is unavoidable.
The legal doctrine of federal laws superseding state laws is called preemption. When a law passed by Congress plainly states that it is intended to preempt state laws, courts have little difficulty applying this express preemption. Somewhat more challenging, though still considered express preemption, is when a federal law does not state an intent to preempt state laws but is such that preemptive intent is implicit. Even when preemption is express, courts must still identify the scope and substance of federal preemption and preserve state laws to the extent possible.
Conflict and Field Preemption
Courts also imply preemption of state law when there is no express preemption. Conflict preemption occurs when state and federal laws are contradictory such that operation of one frustrates the purpose of the other. When such an occasion arises, the federal law will always preempt the state law, providing it is a constitutional exercise of federal authority. Field preemption occurs when Congress regulates a field so completely as to exclude states. However, as in PG&E v. Energy Resources Commission, which involved licensing of nuclear power plants, the Supreme Court found that federal occupation of the field of nuclear plant safety did not prevent states from denying licenses on economic grounds.
Constitutional Values in Conflict
Preemption continues to be a balance of competing constitutional values. First, the founders clearly intended to establish a sovereign national government and, to at least some extent, a set of uniform laws that could hold the country together as one nation. At the same time, the Constitution reflects the compromise of federalism and preserves a significant role for states in the making of laws and governing of individuals. Federal court precedent, especially from the Supreme Court, has evolved significantly over time on the issue of preemption. Most likely, this evolution will continue, but the debate will continue to be guided by the balance between federalism and nationalism.