It would take very little to plunge the blue cities into complete chaos. But if a civil war is waged, everyone will lose to some degree because the economy will suck badly and inflation will go through the roof.
Because of the size of the cities and how lawless they have become, if/when TSHTF, they will descend into chaos rather quickly and many more people will die in the cities than the country as food and water run out and the gangs take over ahd start wholesale murderous rampages.
Additionally, most city folks are terrified of the country and have ZERO idea of how to survive in the wilderness, how to get safe water to drink and where to find food, or how to keep warm and find or build shelter.
If it happens in the fall through spring, it will be even worse as the cold will take an awful lot out.
That all will help make it easier to take care of what’s left even though I would be wary of someone who had the wherewithal to survive. The survivors would be the ones I’d be more concerned about.