What constitutional principle allows the Supreme Court to declare laws unconstitutional?

The principle that allows the Supreme Court to declare laws unconstitutional is judicial review. This power was established in the landmark case Marbury v. Madison in 1803, where Chief Justice John Marshall declared that it is the duty of the judicial branch to interpret the Constitution and void any laws that are found to be in conflict with it. Judicial review thereby serves as a critical mechanism for maintaining the rule of law and upholding the Constitution as the supreme law of the land.

This principle empowers the judiciary to act as a check on the legislative and executive branches, ensuring that any law or executive action adheres to constitutional provisions. The ability to review and overturn laws that violate the Constitution protects individual rights and maintains the intended balance of power between the branches of government.