Something for the USMC to lay waste to!
Yeah, I wouldn't count on it. Marine officers have a pretty strong sense of constitutional obligation.
A "shadow" government is common in a lot of countries. They have no legal power, but serve as a way to organize opposition on various issues. So the Shadow Defense Minister will be their party's lead spokesmen on defense issues, etc..
But, this effort by the Dems will fail. They are too fragmented, and there isn't an equivalent to a consensus opposition leader that exists in parliamentary democracies.