This great principle is that the Constitution and the laws made in pursuance thereof are supreme; that they control the Constitution and laws of the … - John Marshall

Whether a law be void for its repugnancy to the Constitution, is, at all times, a question of much delicacy, which out seldom, if ever, to be decided in the affirmative, in doubtful case. … But it is not on slight implication and vague conjecture that the legislature is to be pronounced to have transcended its powers, and its acts to be considered as void. The opposition between the Constitution and the law should be such that the judge feels a clear and strong conviction of their incompatibility with each other.

[T]he constitution controls any legislative act repugnant to it. . . . It is emphatically the province and duty of the judicial department to say what the law is. . . . So if a law be in opposition to the constitution; if both the law and the constitution apply to a particular case, so that the court must either decide that case conformably to the law, disregarding the constitution; or conformably to the constitution disregarding the law; the court must determine which of these conflicting rules governs the case. This is the very essence of judicial duty. . . . Those then who controvert the principle that the constitution is to be considered, in court, as a paramount law, are reduced to the necessity of maintaining that courts must close their eyes on the constitution, and see only the law. This doctrine would subvert the very foundation of all written constitutions . . . It would be giving the legislature a practical and real omnipotence . . . The judicial power of the United States is extended to all cases arising under the constitution.

The government of the Union, then, … is, emphatically, and truly, a government of the people. In form and in substance it emanates from them. Its powers are granted by them, and are to be exercised directly on them, and for their benefit.