Thomas Jefferson:
First Inaugural Address said,
"If there be any among us who would wish to dissolve this Union, or to change its republican form, let them stand undisturbed as monuments of the safety with which error of opinion may be tolerated where reason is left to combat it."
letter to John C. Breckinridge,Aug. 12, 1803):
“…Besides, if it should become the great interest of those nations to separate from this, if their happiness should depend on it so strongly as to induce them to go through that convulsion, why should the Atlantic States dread it? But especially why should we, their present inhabitants, take side in such a question?…The future inhabitants of the Atlantic & Missipi [sic] States will be our sons. We leave them in distinct but bordering establishments. We think we see their happiness in their union, & we wish it. Events may prove it otherwise; and if they see their interest in separation, why should we take side with our Atlantic rather than our Missipi descendants? It is the elder and the younger son differing. God bless them both, & keep them in union, if it be for their good, but separate them, if it be better.”
letter to To Dr. Joseph Priestley, Jan. 29, 1804
“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 & descendants as those of the eastern, and I feel myself as much identified with 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 & 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.”
letter to Secretary of War 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.”
letter to former U.S. Attorney General Richard Rush
“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.”
At Virginia's ratification convention, the delegates said,
"The powers granted under the Constitution being derived from the People of the United States may be resumed by them whensoever the same shall be perverted to their injury or oppression.
Federalist Paper 39, James Madison
The father of the Constitution, cleared up what "the people" meant, saying the proposed Constitution would be subject to ratification by the people, "not as individuals composing one entire nation, but as composing the distinct and independent States to which they respectively belong."