You’re dreaming or watching too many movies.
Whatever the Domestic Security Force does.... will be in accordance with the current law. Therefore, the military will have no reason to intervene on anyone’s behalf.
The way I approach this in my new novel is to split the military. During a terrible depression, the president's utopian brownshirts organize a constitutional convention, and the new constitution enshrines virtual communism. But half of the nation rejects the new constitution as false and invalid. This causes the loyalty of the military to be divided between the original and the new consitution. That's when the country comes completely unglued. Some regions will be loyal to the president and the new constitution, other regions will reject his authority. It's only fiction, but it's a way we could see the military fracture, and then all bets are off. There's a long excerpt here that details the bogus constitutional convention, and how this could come to pass.
At first anyway. But will it be in accordance with the Constitution?
Meanwhile Obanut is going to cut the *base* military budget (exclusive of cutting funding for the former War on Terror) by over 10% for FY2010. Yea they are going love him.