If we do have a second civil war, it would more likely be like Spain 1936-39, as opposed to North vs. South 1861-65. While conservatives are strongest in the South, lower Midwest, and the Plains and Rockies, you can find patriotic people even in large, liberal cities like New York and San Francisco. Likewise, there are leftist adherents even in the deepest red states like Oklahoma. In 1861-65, Copperhead supporters were numerous only in areas adjacent to Confederate territory or around New York, due to mercantile interests and Irish immigrants. Likewise, Union supporters were significant only in areas like east Tennessee, where slavery was almost unknown, and the Texas Hill Country, with its German immigrants.
The nationalist and republican factions in Spain were coalitions, with the former a mix of "throne and altar" traditionalists and fascists and the latter a combination of Communists, democratic socialists, and anarchists. Something like that would occur if we had a civil war. The Spanish Civil War also had an ethnic elements, with nationalist Basques and Catalans generally supporting the leftist republicans. We would have the same issue with blacks and Hispanics leaning to the leftist coalition.