Martial law has happened before. It can happen again.
And yes, he does circumvent the Constitution and has said in the past that its an impediment to what he wants to do as well as saying that if Congress doesnt respond or act the way he wants them to that hed do it on his own.
I forget which speech it was, (its posted here in the non firearm politics section) he said hed like to consolidate powers to the executive branch. Whiskey Tango Foxtrot? He said it would streamline the decision making process, that the way it is is in efficient. I'm paraphrasing... Point is, his remarks indicate he feels the Contsitution stands as a road block to him doin what he wants. He has in fact said he wants to "fundementally" change the country.
Read the book Philip Drew Administrator.