I drew up a couple of possible maps of my recommendations for the split. The U.S. would actually become three countries, since the whole two countries split with another country between them just hasn't worked out that well (see Pakistan and Bangladesh, for a classic example). At one point, I had thought it would be best to go state by state, and in some ways that would be simpler, but really quite a few states ought to be split -- both Illinois and Indiana need to be split, and Morgantown, WV ought to be appended to the Pittsburgh agglomeration, but the rest of WV really needs to go with the rest of the red states. I've heard that both Oregon and Washington State are quite different when looking at the eastern and western parts of the state.
While it is extremely tempting to call the red states Trumpistan, that is just an artifact of a particularly bitter election season. I think it does represent traditional values for better and worse, and maybe it would just be called The Heartland. Dallas would be a logical capital and would help keep Texas from breaking away on its own. I don't quite know how Alaska would fit in, but it would probably be a colony of sorts.
The third country would be comprised of the Northeastern corridor extended down to at least the Research Triangle in NC and then most of the midwestern cities, though Cincinnati and Indianapolis are excluded on purpose. Coming up with a name is particularly difficult, though given that most of the 13 colonies would be incorporated, it might just be called Old America. While New York would dearly love to be the capital, it might be best to migrate the capital back north (from D.C.) to Philadelphia as a kind of compromise. Or even Pittsburgh for that matter, as it would be more central. (Alternatively, Toronto might be willing to join this new country if it was named capital...)
The main differences between the maps are just how to draw the line between The Heartland and Old America. The second map is a bit more expansive and includes Charleston, as well as the Quad Cities. Maybe Fargo really belongs in The Heartland, though this list suggests Fargo would fit within Old America.
No question there are still difficult questions to resolve, especially what about Colorado, specifically Denver, Boulder and Aspen. Could they be treated a bit like West Berlin back in the day? Austin would also stick out like a sore thumb under this realignment. Notice that I am not nearly as concerned about Florida, as most of the liberal-leaning cities there will be under water in 50 years (or less).
Aside from the difficulty in redrawing maps and property swaps between the new countries, as well as the excitement of each country getting to establish a new Constitution, it would be interesting if there would have to be a resettlement program. It would just be far too painful to be in a liberal or semi-liberal enclave (such as Oxford, Mississippi) and know that you would never in your lifetime have even a liberal President (let alone Senator) and vice versa for conservatives in Pacificanda or Old America.
In any case, feel free to comment about cities that seem to be in the wrong place and need to be moved from one country to another. Note that I am going to be fairly strict about keeping the countries contiguous. If another arrangement, such as four or five countries, makes even more sense, do let me know.