<P> In the United States, judicial review is the ability of a court to examine and decide if a statute, treaty or administrative regulation contradicts or violates the provisions of existing law, a State Constitution, or ultimately the United States Constitution . While the U.S. Constitution does not explicitly define a power of judicial review, the authority for judicial review in the United States has been inferred from the structure, provisions, and history of the Constitution . </P> <P> Two landmark decisions by the U.S. Supreme Court served to confirm the inferred constitutional authority for judicial review in the United States: In 1796, Hylton v. United States was the first case decided by the Supreme Court involving a direct challenge to the constitutionality of an act of Congress, the Carriage Act of 1794 which imposed a "carriage tax". The Court engaged in the process of judicial review by examining the plaintiff's claim that the carriage tax was unconstitutional . After review, the Supreme Court decided the Carriage Act was constitutional . In 1803, Marbury v. Madison was the first Supreme Court case where the Court asserted its authority for judicial review to strike down a law as unconstitutional . At the end of his opinion in this decision, Chief Justice John Marshall maintained that the Supreme Court's responsibility to overturn unconstitutional legislation was a necessary consequence of their sworn oath of office to uphold the Constitution as instructed in Article Six of the Constitution . </P>

If the supreme court declares a law unconstitutional they are exercising what principle