I think they would have devolved to a third world nation if successful in removing themselves from the USA because of slave revolts, slavery, reduced trade, regional wars, corruption, etc. Bill Clinton ran Arkansas like a banana republic even as part of the USA; one can only imagine if it were a sovereign state! It would look like Red Square China, but with Clinton's stupid face staring back at you from every corner.
How are banks, software, and manufacturing a “natural resource?” If there were 2 countries, you would have been led by Mass, NY, and Mi socialist ideology.
In the late 1800’s, NYC and Chicago were full of Eastern European Socialists and Anarchists. Without the south, they would have had more influence with the Electoral College. Then you have the northern plains commie farmers of Mi, Ia, Il who wanted to run socialist populists for national government.