Article VI, Clause 2:
This Constitution, and the Laws of the United States which shall be made in Pursuance thereof; and all Treaties made, or which shall be made, under the Authority of the United States, 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.
Until roughly the New Deal, as recited above, the Supreme Court applied a doctrine of
dual federalism, under which the Federal Government and the states were separate sovereigns, each preeminent in its own fields but lacking authority in the other's. This conception affected preemption cases, with the Court taking the view, largely, that any congressional regulation of a subject effectively preempted the field and ousted the states.1 Thus, when Congress entered the field of railroad regulation, the result was invalidation of many previously enacted state measures. Even here, however, safety measures tended to survive, and health and safety legislation in other areas was protected from the effects of federal regulatory actions.