Federalism as a safeguard of the separation of powers

States frequently administer federal law, yet scholars have largely overlooked how the practice of cooperative federalism affects the balance of power across the branches of the federal government. This Article explains how states check the federal executive in an era of expansive executive power and how they do so as champions of Congress, both relying on congressionally conferred authority and casting themselves as Congress's faithful agents. By inviting the states to carry out federal law, Congress, whether purposefully or incidentally, counteracts the tendency of statutory ambiguity and broad delegations of authority to enhance federal executive power. When states disagree with the federal executive about how to administer the law, they force attention back to the underlying statute: Contending that their view is consistent with Congress's purposes, states compel the federal executive to respond in kind. States may also reinvigorate horizontal checks by calling on the courts or Congress as allies. Cooperative federalism schemes are a more practical means of checking federal executive power than many existing proposals because such schemes do not fight problems commentators emphasize-a vast administrative state, broad delegations, and polarized political parties-but rather harness these realities to serve separation of powers objectives.