If Texas secedes, it becomes part of Mexico (again) almost immediately and the US citizens who stay would likely have naturalized citizenship and vote in a completely different election.
To be honest, I think secession is just a bunch of lies because they're too chicken to realize what they stand against if they stand alone, not to mention how it could end up coincidentally turning into an umforgettable moment of human rights victories once Texans (the nice ones) start voting for better leadership in elections.
If Mexico attempted to annex Texas, it would be a failure of government. There is nothing that the 1.294 million people who reside in the 31 states that currently comprise the United Mexican States could possibly gain, and a lot they could lose, from adding Texas to their ranks.