as "the power to veto encroaching laws . . . or even to disregard them when they are unconstitutional." Id, at 906 (Scalia, J., concurring); see also Youngstown Sheet & Tube Co. v. Sawyer, 343 U.S. 579, 635-38 (1952) (Jackson, J., concurring) (recognizing existence of President's authority to act contrary to a statutory comma