Certainly you would have been impeached, and rightfully so. You take an oath to uphold the laws, you break the laws, you get impeached and convicted. That is as it should be. That would be the courageous part of making such a move. As it will turn out though, this is not the disaster that it could have been or was represented. It is bad enough, mind you, but it could have been far, far worse.
I would not have been impeached (not with Delay running things), and declaring martial law in a national emergency is on the books. Sure if I had known that the storm would suddenly deflect and degrade at the last moment, I would have just bitched slapped Blanco on TV, early and often. But I didn't know that. My best information is that the odds were high that NO would have been blown apart, and then anyone remaining alive, drowned, or most of them, and with that material risk, I would have just have done it, and accepted the consequences. Of course Bush didn't know that, because one of his flaws, is that he lacks a certain intellectual curiousity, and is not very good at moving fast, when fast is the only speed that is safe.