My interpretation of Thomas Jefferson’s writings is that he favored freedom of secession but he thought it was not needed during his lifetime. He wrote about “scission.”

‘The Missouri constitution is recently rejected by the House of Representatives. What will be their next step is yet to be seen. If accepted on the condition that Missouri shall expunge from it the prohibition of free people of colour from emigration to their state, it will be expunged, and all will be quieted until the advance of some new state shall present the question again. If rejected unconditionally, Missouri assumes independent self-government, and Congress, after pouting awhile, must recieve them on the footing of the original states. Should the Representative propose force, 1. the Senate will not concur. 2. were they to concur, there would be a secession of the members South of the line, & probably of the three North Western states, who, however inclined to the other side, would scarcely separate from those who would hold the Misisipi from it’s mouth to it’s source. What next? Conjecture itself is at a loss. But whatever it shall be you will hear from others and from the newspapers. And finally the whole will depend on Pensylvania. While she and Virginia hold together, the Atlantic states can never separate. Unfortunately in the present case she has become more fanaticised than any other state. However useful where you are, I wish you were with them. You might turn the scale there, which would turn it for the whole. Should this scission take place, one of it’s most deplorable consequences would be it’s discouragement of the efforts of the European nations in the regeneration of their oppressive and Cannibal governments.

Amidst this prospect of evil, I am glad to see one good effect. It has brought the necessity of some plan of general emancipation & deportation more home to the minds of our people than it has ever been before.’
-TJ, 1820