Much of the US air force would go over to the Free States and much of the aviation industry is in the south and midwest. The bigger problem for the free States is not air superiority, I think that is draw, the South/Mid Free States will have trouble getting up to speed with armor. The North does have the edgy as all tanks are now made in Ohio.
A future civil war would be more like the Spanish Civil War of the 1930s rather than our own War between the States. Even in the reddest of areas, like the Bay Area and the Northeast megalopolis, there are many conservative sympathizers who would in effect be a "fifth column" for the secessionists. Likewise, there are college towns, large cities, and minority controlled areas, like the Mississippi Delta and South Texas, in the secessionist states, which would be sympathetic to the Washington regime. During the 1861-65 periods, the rival governments used harsh measures to suppress dissent: the North imprisoned numerous Copperheads and censored antiwar newspapers; secession supporters in the South lynched Unionists, especially in Texas. Both sides used the draft to fill the ranks of the military.
You might have a peaceful parting of the ways, as happened with Czechoslovakia. The worst of scenarios would entail extreme bloodshed, with, for the first time in over 200 years, America being open to foreign intervention. It is not impossible to imagine the Russians and the Chinese supporting the secessionists and the Europeans and the Japanese supporting the Washington regime.