It would be "Article 6" of the US Constitution that proclaims the constitution as the highest law in the land, since this is one of the main tenants of federalism, in which the central government and the states share power.
Explanation:
This Law, and the laws of the United States which shall be made in pursuance thereof; and all arrangements made, or which shall be made, under the government of the United States, shall be the supreme law of the state; and the authorities in every state shall be bound thereby, anything in the Constitution or laws of any State to the contrast not withstanding.
