"Contrary to Justice Souter’s suggestion, Gibbons did not exempt the commerce power from this cardinal rule of constitutional law [that the Court is the "ultimate expositor of the constitutional text"]. His assertion that, from Gibbons on, public opinions has been the only restraint on the congressional exercise of the commerce power is true only insofar as it contends that political accountability is and has been the only limit on Congress’ exercise of the commerce power within that power’s outer bounds…. Gibbons did not remove from this Court the authority to define that boundary." C.J. Rehnquist, United States v. Morrison, 529 U.S. 598, 616 (2000).