"Chief Justice Marshall . . . recognized the unique position of the Executive Branch when he stated that '[i]n no case . . . would a court be required to proceed against the president as against an ordinary individual.'”
"[T]hese principles do not mean that the 'President is above the law.' Rather, they simply acknowledge that the public interest requires . . . protecting the Executive Branch from vexatious litigation that might distract it from the energetic performance of its constitutional duties."