Supremacy Clause

Article VI, Clause 2 of the United States Constitution, known as the Supremacy Clause, establishes the U.S. Constitution, federal statutes, and U.S. Treaties as "the supreme law of the land." The text provides that these are the highest form of law in the U.S. legal system, and mandates that all state judges must follow federal law when a conflict arises between federal law and either the state constitution or state law of any state.

The "supremacy clause" is the most important guarantor of national union. It assures that the Constitution and federal laws and treaties take precedence over state law and binds all judges to adhere to that principle in their courts. - United States Senate

The Supremacy Clause only applies if Congress is acting in pursuit of its constitutionally authorized powers. Federal laws are valid and are supreme, so long as those laws were adopted in pursuance of—that is, consistent with—the Constitution. Nullification is the legal theory that states have the right to nullify, or invalidate, federal laws which they view as being unconstitutional; or federal laws that they view as having exceeded Congresses’ constitutionally authorized powers. The Supreme Court has rejected nullification, finding that under Article III of the Constitution, the power to declare federal laws unconstitutional has been delegated to the federal courts and that states do not have the authority to nullify federal law.