A national divorce along party-lines is impossible, because everyone would instantly recognize that it would be worse than what we have now.

Republicans and Democrats are geographically split along rural/urban lines, not state lines. Most major urban centers are Democrat strongholds, whereas most rural areas and smaller towns are Republican. The DSA would be a geographically-disconnected confederation of oversized city-states. The RSA would be all the land in between those city-states.

If there is a break-up of the USA, it would not be along Republican/Democratic lines. It would be according to geography first-and-foremost (e.g. Oklahoma would want ocean access through Texas). Moreover, the Democratic and Repbulican parties would be quickly supplanted by other parties. The current politics in the USA is the product of US Federal policies, not rationality or practicality.