Bush's only defense
I think most would concede that a President has and should have the authority to selectively declassify national security secrets if doing so is necessary to protect national security.
What Bush did was selectively delassify national security secrets in order to protect himself against a political scandal.
Therefore, Bush's only defense is to argue that his political security is equivalent to the national security of the United states.
That kind of argument hasn't been made since the time of kings and emperors.