Who has the authority to declare a law unconstitutional?

The authority to declare a law unconstitutional rests with the Supreme Court. This power is established through the principle of judicial review, which was articulated in the landmark case of Marbury v. Madison in 1803. Judicial review allows the Supreme Court to evaluate the constitutionality of legislation and executive actions. When the Court finds a law that conflicts with the Constitution, it has the authority to nullify that law, thus ensuring that all laws adhere to the framework established by the Constitution.

The roles of the President, the Senate, and the House of Representatives do not include the power to declare laws unconstitutional. The President can veto legislation and, through executive orders or actions, can influence how laws are applied, but has no judicial authority. Similarly, while the Senate and the House can create and amend laws, they cannot adjudicate on their constitutionality—that's reserved for the judicial branch, specifically the Supreme Court. Therefore, only the Supreme Court has the final say in interpreting the Constitution and determining the legality of laws passed by Congress and actions taken by the President.