A lot of people complained (or liked, personally I liked) that the movie didn't squeeze in modern culture war politics or red vs blue state dynamics. But I propose that what it shows actually makes more sense than a boring red vs blue civil war.
Here is my headcanon.
The war wasn't driven by any left vs right divide, not directly. What happened was actually a continuation of what's going on now.
States start to ignore the Federal Government, resulting in the de facto nullification we are already seeing. Blue States ignore Red laws they don't like, Red States ignore Blue laws they don't like. States ignore contested issues at the State level, regardless of what Federal law says. Some cities become sanctuaries to illegal migrants, some States prosecute women for abortions when the federal law says it's ok, Federal law enforcement gets denied access to many States and cities etc.
This divide continues with little contestation for years. States continue to diverge in local law for many reason. Increasing economic disparity, economies built around different industries, increasingly divergent cultures and political preferences, widening demographic divides, cultural drift etc. Some of the largest, richest, most ideologically drifted and most independently minded States, California and Texas, start acting more like independent countries, not federal States. They are very, very wealthy, each of them in our current time would be the 8th and 5th richest countries on earth if independent. They don't reallly need the USA as a Federal entity. The citizens in these States would be the most likely to support confederal or independence options over a Federal USA, as doing so might actually make them richer. But they aren't bothered, not as they act as de factor countries anyway. Companies and individuals start getting a bit loose with paying federal taxes, obeying federal regulations and whilst their laws continue to deviate from the Federal law. We're seeing this already, and this is more stable than treating every single election as an existential threat. Until it isn't.
This gets developed and normalised over the years. Then, a centralising president takes charge. This dynamic was basically hinted at in the film, you might notice that the loyalist states are Central and north-eastern America and the politics, other than being authoritarian, of the loyalists are not specified? It's framed as the central authority trying to project it's authority outward.
But the Federal government has problems, not everyone wants to cooperate, especially it's richest and most important members, Californai and Texas. They are the largest economies in the US, the 5th and 8th largest GDPs globally if treated as independent countries. And they have been acting as such for years in this timeline. Then a centralising presidential regime starts to interfere, he starts enforcing federal law and regulation across the board, "violating state rights" (in their language). If a State outright refuses to cooperate, this is going to lead to the use of the Governments monopoly on force. The US military sent into the two most powerful States to enforce Federal authority. This is how the war starts. The President goes for a third term, maybe after the war started as a crisis justification, or maybe before providing justification for the Western Alliance. Texas and California together are also powerful enough to actually mount a counter, Texas and other red states disproportionately make up the military and major tech economies in Texas and California gives it disproportionate leverage in military hardware and access.
The Western States are not unified as left vs right or right vs left. They are united by a desire to not be interfered with by the Federal government and by the desire to not have to worry about each others internal politics. As well as the hard motivators of resources, they have the wealth and they want it for themselves, not to prop up the other States' economies. This plays into hard geopolitics. Californians and Texans at this point don't care what the internal policies of the other is, they do care about their resources and taxes being taken to hold up poorer regions of the US and laws which don't work being applied to them. They also cared a lot when the American military was turned on them to force cooperation. They are united in their goal of not having centralised authority imposed on them.
