In an August 12, 1803 letter to John C. Breckenridge Jefferson addressed the issue of New England secession by saying that if they seceded, “God bless them both, & keep them in the union if it be for their good, but separate them, if it be better.” On June 20, 1816, Jefferson wrote to a Mr. W. Crawford that “If any state in the Union will declare that it prefers separation . . . to a continuance in the union,” then “I have no hesitation in saying, ‘let us separate’” (The Writings of Thomas Jefferson, vol. 15, p. 27). Jefferson believed that the right of secession was absolutely necessary if America was to avoid tyrannical government. (And Robert Schlesinger hasn’t the foggiest idea of what he is talking about).
John Quincy Adams believed that if a state or states wanted to secede, then “a more perfect Union” could be formed “by dissolving that which could no longer bind . . .” (John Quincy Adams, The Jubilee of the Constitution, p. 66). In Democracy in America (p. 381) Alexis de Tocqueville observed that “The Union was formed by the voluntary agreement of the States; and in uniting together they have not forfeited their nationality . . . . If one of he states chooses to withdraw from the compact, it would be difficult to disprove its right of doing so, and the Federal Government would have no means of maintaining its claims directly either by force or right.”
via The American Tradition of Secession by Thomas DiLorenzo.