One of the benefits of federalism is the checks and balances
between the two systems of government existing in a federal system—that of the
states and that of the federal government. That is to say, federalism can be
thought of as a governmental system that contains two systems of
government—that of the states and that of the federation. Either of these
systems can go too far, and the other system should have the wherewithal to
pull the other back without compromising its viability. This is why the
consolidation of power in one system (e.g., the U.S. Federal Government)
compromises the viability of a federal system at least with respect to its
checks and balances. One other point: the two systems in a federal system are
on the same level; that is, one is not “above” the other. Hence, the supremacy
clauses in the E.U. and U.S. refer only to competencies or domains assigned to
the federal government.
The full essay is at "Essays on the E.U. Political Economy," available at Amazon.