Word has is that some Texicans are talking secession. That's been tried. Some New England states balked at the War of 1812 and made noises about nullification and secession, but they thought better of it. South Carolina made a fuss about secession in 1832 or thereabouts, but Andrew Jackson blew their arguments out of the water. We even had a bloody war over the issue back in the 1860s. The secessionists lost. Whatever the Founders may have intended with respect to secession or whatever Constitutional argument one might have had in favor of secession the fact is that the issue has been decided in favor of perpetual union.
That's not to say that circumstances might not arise under which secession would be possible. A wekened federal government, widespread sentiment in favor of dissolution of the union, alliances by traitors with foreign powers, or what have you could make secession a reality.
In the case of Texas, I would support its secession. I'd like to see it expelled from the union if it won't secede. Let's amend the Constitution to expel Texas. We should, in all fairness, cede Texas back to Mexico from which we stole it in the 1840s as a token of reparations to that country.