For there to be a civil war, both sides need an army. The US president will have the US army, and the other side might as well have nerf guns. The civil war would last an afternoon.
If there's going to be anything, it's an increase in domestic terrorism.
That's what's going to suck. The war might only last a couple months but I can't imagine months of bombings and violent demonstrations in every city across the U.S.
I think you may have added the "us gov" part from inference, vigilante citizens and vigilante cops have bombed American cities and citizens in the past, and will again.
The US has bombed its own citizens twice. First in 1950 it bombed the Puerto Rican towns of Jayuya and Utuado, then in 1985, the Philadelphia police dropped a bomb on a residential neighborhood while targeting MOVE.
If we were to go down the road of military and police violence against US citizens it would be a much longer list.
Furthermore, the American Civil War was precluded by several decades of escalating tension between the states, not between parties in the federal government. The legal and organizational attributes of a state also served to enable separatist states to even attempt to raise an army (this was by design). Bubba and his buddies will, as you said, be armed with nerf guns comparatively
It would be a guerilla collective against a conventional force that's consistently failed against guerilla tactics.
There's no way to know how long it'd last or how it'd impact politics, but targeted acts of terrorism in cities would likely become a more common tragedy. It would pretty much gaurantee US states increasing their police forces and personal rights eroding, and I'm not looking forward to that.