U.S. constitutional law

U.S. constitutional law "deals with the interpretation and implementation of the United States Constitution." While the U.S. Constitution is widely regarded as a document of much wisdoms, its Framers simply could not foresee all the contingencies over more than two centuries.

In practice, the most important interpreter of constitutional law is the Supreme Court of the United States (SCOTUS). SCOTUS most relies on the doctrine of stare decisis or precedent in earlier Supreme Court and other U.S. court rulings. It will also consider, as informative sources, the decisions of state courts, of treaties and international bodies, and foreign courts, as well as works of legal scholarship.

Supremacy Clause
Based on Article VI of the Constitution, SCOTUS ruled, in the 1819 case of McCulloch v. Maryland,, that "state governments and officials cannot take actions or pass laws that interfere with the Constitution, laws passed by Congress, or treaties."