I think New Mexico is the state most likely to move as a state. The demographic maps of Texas show a big difference between the big-bend and border areas, and the rest of Texas. So I would first expect county-by-county changes that lead to ethnic cleansing in souther Texas.
I think public schools offering instruction in Spanish with English as an elective secong language will be a powerful forcing mechanism to get the "Anglo" minorities to either leave or acclimate to Latino Dimmhi status.
They owuld be the first to put down such attempts...assisted by their Norte Americano brothers and sisters in the rest of Texas.
I am relatviely confidant that there will be no county by county break away in Texas in the forseeable future. In the case of Medellin, Texas pretty much showed what it thought of International, World Court, UN law with Mexico influencing. I cannot speak for New Mexico.