The U.S. Constitution itself answers this in Article VI, Clause 2, known as the Supremacy Clause. It says that:

“This Constitution, and the Laws of the United States which shall be made in Pursuance thereof; and all Treaties made, or which shall be made, under the Authority of the United States, shall be the supreme Law of the Land…”

So, in plain language, the “supreme law of the land” is:

  • The U.S. Constitution.
  • Federal laws made in line with the Constitution.
  • Treaties made under U.S. authority.

All of these together outrank any conflicting state constitutions or state laws.