It wouldn't be quick or pretty. Imagine Russian Civil War post-1917, with the cities confiscating food from the surrounding countryside. Our Blue cities would use their police forces and SWAT teams and fragmentary Guard units as a cadre to form and equip militias of their own to break out and seize their needs, and gain control of critical infrastructure like reservoirs and ammo factories, issuing worthless vouchers for what they take, no doubt. The cities have some advantages in the conflict, such as interior lines and the offensive and initiative (Since the Red forces would have no interest in breaking into the cities and taking them, that means the Blues could choose the time and place to focus their forces). The Red forces have some strategic disadvantages like how to redistribute food without using distribution nodes that would fall into Blue territory, after all a tomato farmer may have truckloads of tomatoes, but what does he do for bread, meat, and fuel? How would Red forces deal with refugees fleeing the cities? Without the prewar integrated economic might of the country, their ability to feed and house millions of refugees would be nonexistent, and attempting to handle them would simply cripple the Red forces logistically, leaving them vulnerable to defeat piecemeal. A large percentage of refugees would be Blue loyalists, with many agents and organizers mixed in, with Loyalists just fleeing from necessity, with no desire to change their views. This would quickly lead to large-scale atrocities on both sides, of course whatever remained of the media would focus on atrocities against the Blue population because that's where the media is and that is their power base. Alternatively it could be a low-grade lethal truce with some necessary trade back and forth in commodities, mixed with thousands dead from snipers and occasional mass atrocities, more like Yugoslavia than the Russian Civil War.
Either way, it's so bad it won't happen. Going down that path for either side means buying into a sudden and protracted self-inflicted hardship without a clear path to any gain, which is not something Americans have shown a lot of stomach for in recent times, so the whole thing will remain just internet blather.
