They can talk about it all they want, but they can't seceed, regardless of their rhetoric.
Read the following:
The five to three decision, read on April 15, 1869, by Chief Justice S. P. Chase, held the Union to be indestructable and, thus, not dissoluble by any act of a state, the government, or the people. The court, therefore, repudiated the doctrine of state sovereignty, but it clearly supported the federal in contradistinction to a consolidated system of government, for the decision continues: "But the perpetuity and indissolubility of the Union, by no means, implies the loss of distinct and individual existence or of the right of self-government by the states."
And
Texas v. White, 74 U.S. 700 (1869) was a significant case argued before the United States Supreme Court in 1869. The Court held (in a 53 decision) that Texas had remained a state of the United States ever since it first joined the Union, despite its joining the Confederate States of America and its being under military rule at the time of the decision in the case. It further held that the Constitution did not permit states to secede from the United States, and that the ordinances of secession, and all the acts of the legislatures within seceding states intended to give effect to such ordinances, were "absolutely null".
Unfortunately, the practice of slavery is inconveniently unavailable for the Union to use as their "just cause" this time.
Can one party to a contract unilaterally reinterpret the meaning of the contract years after it is signed? How many of the states which initially created the current U.S. government by ratifying the Constitution in 1787-1788 believed they were taking an irrevocable step?