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Why can U.S. courts declare a law unconstitutional?

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President Lyndon B. Johnson signs the Medicare Bill at a table beside former President Harry S. Truman, with Lady Bird Johnson and others standing behind them. — Legislation Former U.S. president Harry S. Truman (right) looking on as U.S. Pres. Lyndon B. Johnson signs the Medicare bill at the Harry S. Truman Library and Museum in Independence, Missouri, July 30, 1965.

U.S. courts have the authority to declare a law unconstitutional through the doctrine of judicial review, which establishes the judiciary as the ultimate arbiter of the U.S. Constitution.12

This power is rooted in the principle that the Constitution is the supreme law of the land and that it is the duty of the judicial branch to interpret and uphold it. As established in the landmark case Marbury v. Madison (1803), if a legislative act is found to be repugnant to or inconsistent with constitutional principles, the courts must declare it null and void.231

This authority serves as a critical component of the checks and balances system, ensuring that the legislative and executive branches remain within the limits defined by the Constitution. By preventing the government from exceeding its prescribed powers, judicial review acts as a safeguard for limited government and individual liberties. Every judge in the United States is empowered to engage in this constitutional interpretation when a relevant question arises during a genuine legal controversy.34