My concern for following the Constitution is tied to the practical need to make the transition as painless as possible for the average person.
Most Americans say they believe in the Constitution, so it's a good skeleton to use to structure the split. Otherwise you risk a free-for-all that might wind up with a Russia-style Red America spending the next century trying to reclaim the Blue.
I could defend Pacific America from that because we've got about 1,000 miles of desert and mountain between us and the Red heartland. But if the country does fall into a widespread civil war, I suspect it will get international and very bloody.