Good point. With the election of Trump or Hillary in November, the collapse of the federal government is highly probable at which point individual states will simply go their own way or band together in new confederations. In other words, Texas will not have to 'leave' because there won't be anything to 'leave' from.
On the other hand, if Trump or Hillary become the dictators they want to be then the Constitution is essentially nullified and therefore no consent is required for states to 'leave'.
In either scenario, there's probably gonna be a bit of blood spilled.
I consider it most likely a number of states will split off rather than a single state, as the issues against the federal govt are the same in multiple states of like mind. Texas just happens to be the largest and most vocal of those states.
The states that remain will fight vociferously as they will realize they will be 'holding the bag' of the $20T in debt. But there will not be much they can do about it to keep Texas leaving, whether it does it with consent of others or not.
At least the reduced federal govt will not be on the hook for future SS or Medicare for the citizens of those states which had left. That is the cost of leaving, which for my money is well spent.
And, as one reads the news today regarding UK once it made decision to leave the EU, countries will line up eagerly to begin trade with the new nation. Rich in resources and with the temperament of its people, Texas will do just fine and thrive.