A constitutional power is called dormant if it is granted by the constitution but is not currently being exercised. For a variety of reasons Congress may not see fit to exercise power which it has been granted by the Constitution. These dormant powers may be "awakened" whenever Congress chooses to exercise them.
The federal system of the United States presumes that the states possess the governmental powers not taken away from them by the Constitution. In vesting powers in the federal government, the Constitution grants the national government exclusive power over some matters of national scope, but concurrent powers with the states over other matters. When a power granted to the federal government lies dormant, its effect depends on whether it is an exclusive or concurrent power. When the Supreme Court concludes that the subject matter requires uniform national regulation, then the states are not free to exercise the power even though Congress has not legislated in the area. By its silence Congress is presumed to have decreed that the subject shall remain free of regulation. If, on the other hand, the subject of the power admits of locally diverse regulation, as in cooley v. board of wardens (1851), states may legislate until Congress intervenes.
When Congress awakens a dormant power by legislating, its act will preempt inconsistent state laws by force of the supremacy clause of Article VI. However, the courts do not always hold that Congress has preempted state law simply because it has granted power to a federal administrative agency to regulate an area. If the agency has not exercised the power given to it, then the states may yet be free to regulate the subject just as if the power were still dormant.
Glen E. Thurow
Pritchett, C. Herman 1977 The American Constitution, 3d ed. Pages 150–216. New York: McGraw-Hill.