Pick a better statesman, Ida. Jefferson changed his mind about so many things that "serious students of history" can certainly wonder about what view he would have taken. Politicians are like that, and Jefferson in office certainly was a politician.
Jefferson didn't believe the federal government had the right to purchase the lands west of the Mississippi from France, but when it looked like the opportunity would pass he changed his mind. Here's one writer's view of the embargo crisis:
How long could the end of peaceable coercion abroad be supported in the face of economic deprivation, loss of liberty, disobedience to law, division of the Union, and Republican collapse at home? Despite rising opposition, Jefferson stood firmly by the policy. Perhaps he recalled his experience in another crisis, when he, as Virginia's governor, was accused of jeopardizing the safety of the commonwealth by feeble and temporizing measures. To Gallatin, who complained that the embargo could be saved only by new and arbitrary enforcement powers, Jefferson replied, "Congress should legalize all means which may be necessary to obtain its end ," not excluding military force.
That's just one person's view of course. But to say that it's a given that Jefferson would have sat back and done nothing in a secession crisis is to make a questionable assumption.
As did Lincoln:
“Any people, anywhere, being inclined and having the power, have the right to rise up and shake off the existing government, and form a new one that suits them better. This is a most valuable and most sacred right - a right which we hope and believe is to liberate the world. Nor is this right confined to cases in which the whole people of an existing government may choose to exercise it. Any portion of such people that can, may revolutionize, and make their own, of so many of the territory as they inhabit.”
Abraham Lincoln
Thomas Jefferson, while in office, contradicts the pandering political Abe Lincoln.
“The future inhabitants of the Atlantic & Missipi 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.”
http://www.let.rug.nl/usa/P/tj3/writings/brf/jefl159.htm