Both Jefferson and Madison in their writings said that secession should be done with the agreement of both sides, those leaving and those staying.
Both Jefferson (in his Kentucky Resolutions and his Declaration of 1825) and Madison (in his Virginia Resolutions and Report on those same resolutions) explicitly recognized the right of each State, as a party to the constitutional compact, "to judge for itself, as well of infractions as of the mode and measure of redress" (Jefferson quoted). That clearly suggests unilateral action, and does not obviously exclude secession. If the same gentlemen suggested elsewhere that States might find it advisable to obtain concurrence from other parties to the compact, I suppose one might argue that their comments regarding unilateral secession might be debatable, but certainly not "easily refutable" as suggested by the author.
Even Rawle, while claiming secession at will was allowed, admitted that the individual states did not have the power to do so arbitrarily.
That may depend on what one considers to be arbitrary. Rawle observed in his View of the Constitution:
"[I]t is not to be understood, that... [the intervention of the power of the Union] would be justifiable, if the people of a state should determine to retire from the union, whether they adopted another or retained the same form of government, or if they should, with the express intention of seceding, expunge the representative system from their code... It depends on the state itself to retain or abolish the principle of representation, because it depends on itself whether it will continue a member of the Union... To deny this right would be inconsistent with the principle on which all of our political systems are founded, which is, that the people have in all cases a right to determine how they will be governed... The states, then, may wholly withdraw from the Union..." [Chap. XXXI, emphasis mine]
Again, the right of secession (as it was understood prior to the war) was most definitely not "easily refutable"...
;>)
In an 1832 letter to Alexander Rives Madison seems to deny that was his intent. He writes: "But the ability and the motives disclosed in the Essays induce me to say in compliance with the wish expressed, that I do not consider the proceedings of Virginia in ’98-’99 as countenancing the doctrine that a state may at will secede from its Constitutional compact with the other States. A rightful secession requires the consent of the others, or an abuse of the compact, absolving the seceding party from the obligations imposed by it."
Again, the right of secession (as it was understood prior to the war) was most definitely not "easily refutable"...
But in Chapter XXXII "On the Permanence of the Union" Rawle also writes: "The secession of a state from the Union depends on the will of the people of such state. The people alone as we have already seen, bold the power to alter their constitution. The Constitution of the United States is to a certain extent, incorporated into the constitutions or the several states by the act of the people. The state legislatures have only to perform certain organical operations in respect to it. To withdraw from the Union comes not within the general scope of their delegated authority. There must be an express pro- vision to that effect inserted in the state constitutions. This is not at present the case with any of them, and it would perhaps be impolitic to confide it to them. A matter so momentous, ought not to be entrusted to those who would have it in their power to exercise it lightly and precipitately upon sudden dissatisfaction, or causeless jealousy, perhaps against the interests and the wishes of a majority of their constituents.