If this president tried to impose a new constitution unilaterally, they would be impeached and removed for breaking their oath of office. There would be no support for such a move, regardless of who it came from. If they tried to organize a constitutional convention, the proposal would still be dead in the water, but that would be the end of it.
There wouldn't be a war. That would just be silly. Unless the gun nuts just started shooting in attempt to live out their "fight against tyranny!!1" fantasies. There wouldn't be a coup, either-- trampling on the constitution in order to save it sort of defeats the point. The only way the military would enter the picture would be if the president refused to accept the Senate's verdict and had to be physically escorted from the White House.
What if a bunch of protesters in favor of the President were let in by the President armed and ready to defend him to the death? Scary stuff
