Thomas Jefferson, in 1816, wrote about his view of the Union to then Secretary of War (soon to be Sec. of the Treasury) William Crawford, “If any State in the Union will declare that it prefers separation with the first alternative, to a continuance in union without it, I have no hesitation in saying “let us separate." I would rather the States should withdraw which are for unlimited commerce and war, and confederate with those alone which are for peace and agriculture.”
These views were not new to Jefferson. We see this same understanding in 1798, while he was serving as Vice-president as the United States. In that year Jefferson wrote what has come to be known as the Kentucky Resolutions, which were written in response to the Alien and Sedition Acts.
In 1804 he wrote the following in a letter to Dr. Joseph Priestley, “Whether we remain in one confederacy, or form into Atlantic and Mississippi confederacies, I believe not very important to the happiness of either part. Those of the Western confederacy will be as much our children and descendants as those of the Eastern, and I feel myself as much identified what that country, in future time. As with this: and did I now foresee a separation at some future day, yet I should feel the duty and the desire to promote the Western interests as zealously as the Eastern, doing all the good for both portions of our future family which should fall within my power.” This was written because of secession talk due to the Louisiana Purchase.
Again in 1820 Jefferson wrote on this topic again because of sectional friction because of debate over Missouri. He wrote, “The experiment of separation would soon prove to both that they had mutually miscalculated their best interests. And even were the parties in Congress to secede in a passion, the soberer people would call a convention and cement again the severance attempted by the insanity of their functionaries.” This was written former U.S. Attorney General Richard Rush. Notice, Jefferson did not think that secession at this time to be wise nor that it would last long, but he certainly believed it a viable/legal option.
On matters of religion, I differ with Jefferson at many points, because I hold to orthodox Christianity and Jefferson had rejected the Christian faith. On political issues I stand much closer to Jefferson. I am for true constitutional federalism (which was lost in 1865), and also limited government (which was lost in 1865, mostly regained after that for a time and now almost completely disappeared again and forgotten).