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.
The Constitution, laws, and treaties of the United States are as much a part of the law of every state as its own local laws and constitution. Their obligation
is imperative upon the state judges, in their official and not merely in their private capacities. From the very nature of their judicial duties, they would be called upon to pronounce the law applicable to the case in judgment. They were not to decide merely according to the laws or Constitution of the State, but according to the laws and treaties of the United States – ‘the supreme law of the land.’ 1 State courts are bound then to give effect to federal law when it is applicable and to disregard state law when there is a conflict; federal law includes, of course, not only the Constitution and laws and treaties but also the interpretations of their meanings by the United States Supreme Court. 2 Although states may not have to specially create courts competent to hear federal claims or give courts authority specially, 3 it violates the Supremacy Clause for a state court to refuse to hear a category of federal claims when the court entertains state law actions of a similar nature, 4 or sometimes even when it does not entertain state law actions of a similar nature. 5 The existence of inferior federal courts sitting in the states and exercising often concurrent jurisdiction of subjects has created problems with regard to the degree to which state courts are bound by their rulings. Though the Supreme Court has directed and encouraged the lower federal courts to create a corpus of federal common law, 6 it has not spoken to the effect of such lower court rulings on state courts.