The Supreme Court is the final arbiter of what is and what is not constitutional. In other words, if Congress or the President take an action such as President Trump’s Executive Order (EO) which banned entry into the Country of persons from 7 listed countries; notwithstanding valid Visa or legal permanent residence status (i.e., greencard holders), federal Courts have authority to review the EO. If the Court determines that the EO is unconstitutional, then the EO is void. In short, the Courts may effectively overrule the acts and actions of the President and Congress.
You will not find any explicit language in the Constitution stating that the Supreme Court is the final arbiter of what is constitutional. The Founders set up 3 co-equal branches of government: 1) Legislative Branch; 2) Executive Branch; and 3) Judicial Branch. But they did not indicate in the Constitution itself what would or should happen in the event there is a dispute over the actions of one or more branches.
Marbury v. Madison
This concept of judicial review came about in one of the first cases heard by the U.S. Supreme Court. In 1800, Thomas Jefferson, then the Vice-President, ran against the incumbent President, John Adams. Though they had been friends during the founding of the Country, by 1800 they had very different political views and had become bitter rivals. After losing the election in November 1800, Adams was trying to use the last few days of his Presidency to appoint persons favorable to him to judgeships and as justices of peace. Adams nominees had the consent of the Senate and commissions were then prepared for each appointment.
Another founding father, John Marshall was Adams’ Secretary of State. Before Marshall could finish delivering the commissions, Thomas Jefferson was seated as the third President of the United States (U.S.). Jefferson instructed his Secretary of State, James Madison, not to deliver the remaining commissions. But before leaving office, another of Adams last appointments that had been granted with the proper advice and consent of the Senate, was the appointment of his Secretary of State, John Marshall, to be the fourth Chief Justice of the U.S.
William Marbury had been appointed a Justice of the Peace by Adams but his commission had not been delivered. Marbury sued Madison, the new Secretary of State under Jefferson, to force Madison to deliver the commission. In Marbury v. Madison, 5 US 137 (1803), the Supreme Court determined that Marbury did not have a right to the commission and that Congress had exceeded its authority in Section 13 of the Judiciary Act of 1789 by attempting to extend the original jurisdiction of the Supreme Court in conflict with the Constitution.
More importantly, Chief Justice Marshall’s opinion stated that the Constitution, was the “supreme law of the land” and that a “law repugnant to the Constitution is void.” Marshall’s opinion established the concept of “judicial review”. Judicial review allows the Courts to review the actions of Congress, the President and his executive branch agencies.
In Marbury v. Madison, 5 US 137 (1803), the U.S. Supreme Court held that the Constitution was the “supreme law of the land” and that any statute or action of the President or Congress that runs counter to the Constitution is void. This concept of judicial review allows the Courts to review and rule upon the constitutionality of acts and actions by the legislative and executive branches.
Note: Although the Court has authority to review the actions of the President and Congress, a party that opposes such actions must bring a “case or controversy” to the Court in order for the Court to rule. US Const. Art. III, Sec. 2. The Court may not issue advisory opinions nor may it review a legislative or executive action on its own motion unless it has been brought to the Court as a case or controversy.