The enormous powers Congress wields through this clause cannot be fully defined. The Supreme Court has not defined them. Like sovereignty itself, the exercise of its essential powers, even when delegated functionally in government, does not yield to the limits of definition. The decisions of the Supreme Court are not definitions of the power over commerce so much as they are definitions of the particular exercise of the power of Congress within its jurisdiction, with respect to commerce, by the Constitution.[129] For the States also have jurisdiction over commerce. Our knowledge of the boundaries of these two jurisdictions arises from the conflict of laws concerning them.
50. In defining national jurisdiction and State jurisdiction over commerce, two propositions are fundamental:
(1) The Constitution of the United States is the supreme law of the land.[130]
(2) It is the province and duty of the judicial department to say what the law is.[131]
It should be clearly understood that power to regulate commerce is incident to sovereignty. Government—whatever its form—is a delegation of power by sovereignty, and of necessity possesses this power of regulation. The degree or extent of the delegation of the power to regulate commerce marks unmistakably the jurisdiction of the government exercising the power. The analogy is in the taxing power. In our system of dual government—national and State—there are two jurisdictions. The respective States have power over commerce; the United States has power to regulate commerce,—each jurisdiction expressly or impliedly outlined by the Constitution.
51. With slight change in wording, the leading decisions of the Supreme Court on the power of the United States to lay and collect taxes, and its decisions on the subject interpretative of the taxing power of the States, apply, in principle, to their respective powers over commerce:
| If we measure the power of | { | taxation | } |
| “regulating commerce” |
residing in a State, by the extent of sovereignty which the people of a single State possess and can confer on its government, we have an intelligent standard, applicable to every case to which the power may be applied. We have a principle which leaves the power of
| { | taxing the people and property of the State | } |
| “regulating the commerce of the State” |
unimpaired; which leaves to a State the command of all its resources, and which places beyond its reach all those powers which are conferred by the people of the United States on the government of the Union, and all those means which are given for the purpose of carrying those powers into execution. We have a principle which is safe for the States, and safe for the Union. We are relieved, as we ought to be, from clashing sovereignty; from interfering powers.[132]