This would be so much fun to watch. They wouldn't be able to take the US army with them, US army bases would be dismantled, nukes would be removed, Mexico would be like "fuck no" and have no trade with them, and the US, being their former Union, would also give them the finger. Thousands, plug not millions of people would cross borders there to either leave or join the shit hole (because fuck the liberal US government!) causing a huge outflux of knowledge and competence and a huge influx of rednecks.

Let me be clear though: this will never happen, because the politicians calling for this know damn well that they'd be fucked. They're just riling their base with dumb but popular rhetoric.