Marbury v. Madison, also cited in the U.S. v. Nixon decision, was the first time that the Supreme Court ruled that a law could be unconstitutional. It formally established the right of judicial review, i.e., that laws can be reviewed by the judicial branch to evaluate their constitutionality.

In Marbury, the Court ruled that President Thomas Jefferson was wrong in preventing William Marbury, who was appointed by his ex-bestie John Adams, from taking his appointed office as justice of the peace. Unfortunately for Marbury, the Supreme Court also claimed the case that allowed Marbury to bring his suit was unconstitutional, and couldn't force Jefferson to allow Marbury to take his appointment.

This is one of the first cases that addressed the separation of powers among the branches of government. The reason that Marbury v. Madison was referenced in the U.S. v. Nixon was to reaffirm that the Supreme Court has the final word in determining constitutional questions, and to affirm that no man, not even the president, was above the law.

While the cases are the same, and have to do with the jurisdiction of the courts, their outcomes were different. While the Supreme Court didn't force Jefferson to seat Marbury, it did force Nixon to surrender his tapes to the Watergate Committee.