The clause of Article VI of the U.S. Constitution that declares that all laws and treaties made by the federal government shall be the "supreme law of the land."
Article VI, Section 2, of the U.S. Constitution is known as the Supremacy Clause because it provides that the "Constitution, and the Laws of the United States … shall be the supreme Law of the Land." It means that the federal government, in exercising any of the powers enumerated in the Constitution, must prevail over any conflicting or inconsistent state exercise of power.
The concept of federal supremacy was developed by Chief Justice John Marshall, who led the Supreme Court from 1801 to 1835. In McCulloch v. Maryland, 17 U.S. (4 Wheat.) 316, 4 L. Ed. 579 (1819), the Court invalidated a Maryland law that taxed all banks in the state, including a branch of the national bank located at Baltimore. Marshall held that although none of the enumerated powers of Congress explicitly authorized the incorporation of the national bank, the Necessary and Proper Clause provided the basis for Congress's action. Having established that the exercise of authority was proper, Marshall concluded that "the government of the Union, though limited in its power, is supreme within its sphere of action."
After the Civil War, the Supreme Court was more supportive of states' rights and used the Tenth Amendment, which provides that the powers not delegated to the federal government are reserved to the states or to the people, to justify its position. It was not until the 1930s that the Court shifted its position and invoked the Supremacy Clause to give the federal government broad national power. The federal government cannot involuntarily be subjected to the laws of any state.
The Supremacy Clause also requires state legislatures to take into account policies adopted by the federal government. Two issues arise when state action is in apparent conflict with federal law. The first is whether the congressional action falls within the powers granted to Congress. If Congress exceeded its authority, the congressional act is invalid and, despite the Supremacy Clause, has no priority over state action. The second issue is whether Congress intended its policy to supersede state policy. Congress often acts without intent to preempt state policy making or with an intent to preempt state policy on a limited set of issues. Congress may intend state and federal policies to coexist.
However, some federal legislation preempts state law, usually because Congress believes its law should be supreme for reasons of national uniformity. For example, the National Labor Relations Act of 1935 (Wagner Act) (29 U.S.C.A. § 151 et seq.) preempts most state law dealing with labor unions and labor-management relations.
In Pennsylvania v. Nelson, 350 U.S. 497, 76 S. Ct. 477, 100 L. Ed. 640 (1956), the Supreme Court developed criteria for assessing whether federal law preempts state action when Congress has not specifically stated its intent. These criteria include whether the scheme of federal regulations is "so pervasive as to make the inference that Congress left no room for the States to supplement it," whether the federal interest "is so dominant that the federal system [must] be assumed to preclude enforcement of state laws on the same subject," or whether the enforcement of a state law "presents a serious danger of conflict with the administration of the federal program."
