They will never pull it off as long as we have the guns. They know that as well as we do, hence the unrelenting leftist drive for more and more gun laws to disarm us.
We have a lot of guns and it would seem most of the hard military experience. We also occupy a lot of space and tend to less dense areas, that is initially a problem.
There are more guns though, and the world is full of amateurs. They have the initial advantage of tending to higher density populations areas. Depending on how long any such fight lasted and how they used the military knowledge they initially have, it could get really, really bloody.
All this is without knowing who the federals would support. I tried to game it out one time, maybe I should sit down and right that game out, the results weren't pretty.