Sec.A1.AAFREEDOM AND SOVEREIGNTY OF STATE. Texas is a free and independent State, subject only to the Constitution of the United States, and the maintenance of our free institutions and the perpetuity of the Union depend upon the preservation of the right of local self-government, unimpaired to all the States.
Here's part of Article 1, Section 2.
The faith of the people of Texas stands pledged to the preservation of a republican form of government, and, subject to this limitation only, they have at all times the inalienable right to alter, reform or abolish their government in such manner as they may think expedient.
The way I read this, if the Federal Government violates the Constitution or ceases to be a Republican form of government, we can leave.
I think that it's clear that the feds are violating the Constitution (and have been for some time). The big argument will be, do we still have a Republic? If it's proven that we do not, then we can go our merry way.
Now, will they let us go without a fight... ?