While it is true that I talked about preventing the ACW rather than preventing secession, I think the focus by some people here on having a constitutional provision dealing specifically with secession is misguided. I don't think any such provision is likely to be included. (Note that even the Confederate constitution did not say anything explicitly about a right to secession!) Federalists would object to a provision allowing a state the right to secede; it would undermine their whole purpose of creating a stronger central government, especially considering the blackmail power it would give disaffected states. They were certainly not looking for a Union that was *easier* to dissolve than the Confederation! OTOH, any provision specifically denying the right to secede would just be pointed to by Antifederalists as proof of how tyrannical the new Constitution was.
And anyway, even if such a provision were included and all the states ratified it, it wouldn't necessarily prevent secession. Southern states would argue, " "Yes, the Constitution prohibits secession. But it also requires the rendition of fugitive slaves and--at least according to our interpretation and that of the Supreme Court--forbids federal interference with slavery in the territories. Since the North has either violated these provisions or is about to do so, we are not bound by the anti-secession provision. The contract has been so violated by one side that the other side may take it as a nullity." Or instead of relying on a right of secession, they would rely on a "natural" right of revolution. Some did in OTL, in fact, like Senator Alfred Iverson of Georgia:
"The President [Buchanan] may be right when he asserts the fact that no
State has a constitutional right to secede from the Union. I do not myself
place the right of a State to secede from the Union upon constitutional
grounds. I admit that the Constitution has not granted that power to a
State. It is exceedingly doubtful even whether the right has been
reserved. Certainly it has not been reserved in express terms. I therefore
do not place the expected action of any of the Southern States, in the
present contingency, upon the constitutional right of secession; and I am
not prepared to dispute therefore, the, position which the President has
taken upon that point.
"I rather agree with the President that the secession of a State is
an act of revolution taken through that particular means or by that
particular measure....But, sir, while a State has no power, under the
Constitution, conferred upon it to secede from the Federal Government or
from the Union, each State has the right of revolution, which all admit.
Whenever the burdens of the government under which it acts become so
onerous that it cannot bear them, or if anticipated evil shall be so great
that the State believes it would be better off--even risking the perils of
secession--out of the Union than in it, then that State, in my opinion,
like all people upon earth has the right to exercise the great fundamental
principle of self-preservation, and go out of the Union--though, of
course, at its own peril--and bear the risk of the consequences..."
http://www.gutenberg.org/files/15393/15393-h/15393-h.htm