The best explanation for the major source of state government power during the period of dual federalism is the reserved powers given to states by the Tenth Amendment.
During the era of dual federalism (roughly from the founding of the U.S. until the 1930s), the Tenth Amendment, which reserves powers not delegated to the federal government to the states or the people, served as a foundation for state authority and autonomy. This framework emphasizes a clear distinction between federal and state responsibilities, which is a hallmark of dual federalism.