Texas, Oklahoma, eastern Colorado, Kansas, Nebraska, Utah, Wyoming, Idaho, Montana, South Dakota, North Dakota would make a decent country... plenty of minerals, agriculture, and access to the Gulf of Mexico.
I would expect in that scenario, for southern California and New Mexico to break off and become part of Mexico. Arizona, I am not sure of. Washington, Oregon, and northern California would be suitable additions to Canada.
Pockets of heavy resistance will be found here in CO. AZ, likely the same, although it will be awful rough in the beginning for AZ.