I don't have the least doubt that there would be lots of support. Certain groups, and there are many, would be pleased as punch to see Obeyme wipe out city blocks full of "domestic terrorists" like us.
Well yeah, there's always been that segment of the hard left who would gladly see the throat of every patriot cut, but they're a small number of the rank and file left.
A dictatorial President would need near universal approval from the people in the union states to launch military actions against any state that merely changed its political alignment. That kind of broad support just doesn't exist today.
More likely, you'd see an endless amount of gloating and glee in the blue states over the secession of any red state.