You have to realize, that in the US the idea of "Civil War" is highly distorted because our only major example of it was highly regional. That is actually highly atypical in a civil war unless it is in a country with significant cultural or ethnic divisions in the nation (like the former Yugoslavia).
Most scenarios I have seen tend to place it as more of an Urban versus Rural split. Not unlike several of the attempts to split up California. Not regional at all, but spread throughout the nation, with urban areas tending towards one side, and rural areas tending towards the other.
A good example would be the Chinese Civil War. Where the KMT controlled most of the nation, but the industrial areas and large cities supported the CCP. Or the English Civil War, where the King controlled most of the nation, but Parliament controlled London and the large cities.