One reason why the secessionists would have to doubt the likelihood of even a Democratic court siding with them on the legality of secession is that no less a "doughface" than James Buchanan considered secession illegal and indeed IMO gave the best single argument for its illegality: "Such a principle is wholly inconsistent with the history as well as the character of the Federal Constitution. After it was framed with the greatest deliberation and care it was submitted to conventions of the people of the several States for ratification. Its provisions were discussed at length in these bodies, composed of the first men of the country. Its opponents contended that it conferred powers upon the Federal Government dangerous to the rights of the States, whilst its advocates maintained that under a fair construction of the instrument there was no foundation for such apprehensions. In that mighty struggle between the first intellects of this or any other country it never occurred to any individual, either among its opponents or advocates, to assert or even to intimate that their efforts were all vain labor, because the moment that any State felt herself aggrieved she might secede from the Union. What a crushing argument would this have proved against those who dreaded that the rights of the States would be endangered by the Constitution! "
https://en.wikisource.org/wiki/James_Buchanan's_Fourth_State_of_the_Union_Address
Indeed, not all
secessionists thought that secession was legal. Some like Alfred Iverson of Georgia agreed that it was a revolution--and justified it precisely on that basis:
"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. It withdraws from the Federal compact, disclaims any further allegiance to it, and sets itself up as a separate government, an independent State. The State does it at its peril, of course ; because it may, or may not, be cause of war by the remaining States composing the Federal Government. If they think proper to consider it such an act of disobedience, or if they consider that the policy of the Federal Government be such that it cannot submit to this dismemberment, why then they may or may not make war, as they choose, upon the seceding States. It will be a question of course for the Federal Government, or the remaining States, to decide for themselves, whether they will permit a State to go out of the Union, and remain as a separate and independent State, or whether they will attempt to force her back at the point of the bayonet. That is a question, I presume, of policy and expediency, which will be considered by the remaining States composing the Federal Government, through their organ, the Federal Government, whenever the contingency arises.
"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...."
https://books.google.com/books?id=DR8sAAAAIAAJ&pg=PA399