The Border

As you can see from the map, the border between the new Northern United States and Southern United States begins between California and Arizona, travels north between Nevada and Utah, and between Oregon and Idaho. Idaho would be split with the divide occurring by extension eastward of the border between Washington and Oregon. The northern panhandle of Idaho would be absorbed by Montana.

The border then continues eastward between Montana and Wyoming, dropping down between South Dakota and Nebraska, down again between Iowa and Missouri, and down once more to include Illinois. The top-most portion of Indiana, from Gary to Fort Wayne, would be shaved off Indiana, and be absorbed by Michigan.

The border continues on between Ohio and Kentucky, Pennsylvania and West Virginia, and finally ends between Maryland and Virginia. Hawaii would go with the North, and Alaska would go with the South.

There are some trade-offs in this division, in order to keep the two countries contiguous. The North gains northern Idaho, Montana, the Dakotas, and a part of Indiana. The South gets to keep New Mexico and Florida.

This places substantially all of the states that voted a majority of the time for a liberal for president in the last four elections in the North, with the exception of New Mexico. All of the states that voted a majority of the time for a conservative would remain in the south, with the exception of Montana and the Dakotas, which balance out New Mexico.

The North would keep Washington D.C. because of its proximity to Maryland, and continue to use it as the national capital. The South would build their own new capital, perhaps in Richmond, VA. By keeping the two nation's capitals close to each other, they can work more easily through the issues that would arise during the division process. The current sentiment in the existing South is highly anti-government, so presumably they would not mind losing D.C. to the new liberal nation, and they could build a new capital that suits their objectives for government in a manner that pleases them (presumably, scaled-down).

What if I wind up living in the wrong country for me?

Great question. That leads us to....The Treaty.