Two words: Moral authority.
You have more "Moral Authority" than the President?
Boy you really do have a tinfoil hat.
That is enough eh? Pity that sometimes it isn't, although personally I don't think doing what is necessary to defend and protect is immoral, as long as the legislative branch is clued in, as it was. That way, if the legislative branch does not like it, they can impeach and convict, if they disagree with the president's judgment. The courts are out there too, but notice how circumspect they are, about curbing the president when it comes to waging war.