I do not think that that northern disunionism should be totally written off. It was not merely a Garrisonian eccentricity; a considerable number of antislavery northerners did at least toy with it from time to time. Senator John P. Hale of New Hampshire once said: "If this Union, with all its advantages, has no other cement than the blood of human slavery, let it perish." (Quoted in David Potter, *The Impending Crisis 1848-1861*, p. 45) Senator Wade of Ohio stated in 1854 that "I go for the death of slavery whether the Union survives it or not." (Quoted in Brian Holden Reid, *The Origins of the American Civil War* [London and New York: Longman 1996], p. 147) Also, at various times in the 1860-61 crisis, Charles Sumner, Joshua Giddings, Gerrit Smith and other abolitionists advocated the peaceful dissolution of "this blood-stained Union." (Quoted in Kenneth Stampp, *And the War Came* (Phoenix books edition, pp. 247-8)
Still, all this was mostly rhetorical--it was people saying "I would rather have disunion than another cowardly compromise with the Slave Power." Most of the people who said this (a) were much more radical on slavery than most northerners (including a majority of Republicans), and (b) except for the Garrisonians, didn't really believe it *was* necessary to choose between Union and antislavery. It is true that after Buchanan's election in 1856, some northern antislavery radicals concluded that the struggle aginst slavery was hopeless within the Union, and tried to make common cause with the Garrisonians in a "disunion convention" in Worcester, MA in 1857. They sent out invitations to several prominent Republicans--who all turned them down. Even a Radical like Henry Wilson advised the Convention to "leave all the impotent and puerile threats against the Union to the Southern slave propagandists."
https://books.google.com/books?id=Wl38uYb85DgC&pg=PA141 (OTOH, Congressman Edward Wade, Benjamin Wade's brother, was not totally unsympathetic, nor was Amasa Walker
https://en.wikipedia.org/wiki/Amasa_Walker: "Edward Wade agreed with the invitation that slavery and freedom were irreconcilable, and even that, up to 1856, the Union had been a failure. He counseled waiting a while longer to see whether action against slavery could still be taken within the Union, but concluded: 'rather than to give the strength, moral and political, of the people of the Free States, to the extension and pepetuity of slavery, *let the Union perish*. Amasa Walker agreed with Wade, though in less fiery language, that the Union was 'a means and not an end,' and that the question of Union or disunion should always be considered in light of tactics in the overriding contest against slavery.")
The questions are: (a) what would get antislavery northerners to believe that the cause of antislavery within the Union was doomed, and (b) make them a majority in the North--or at least in enough northern states to make a serious movement for secession possible?
The only thing I can think of is a Breckinridge victory in 1860--having the "slave power" win yet another victory will by itself be tremendously embittering--followed by a war in Latin America which northerners would see as a war for slavery, and also by the "second Dred Scott" decision Lincoln had warned about. (Yet a "second Dred Scott" decision *immediately* establishing slavery in the North was unlikely. What was more likely and more insidious was the possibility that the court would establish slavery in the North *gradually* by first recognizing slaveholders' rights briefly to pass through northern states with their human "property" and then step by step expanding that right to one of staying there with the slaves indefinitely--and perhaps even buying and selling them. What worried Lincoln was that the gradualness of the process--combined with Douglas' public moral indifference to slavery and view that a Supreme Court decision was a "Thus saith the Lord" that cannot be questioned--would mute northern outrage.)
More likely, even in the event that Breckinridge won in the House in 1860 (or there was a deadlock in the House so that Breckinridge's "doughface" running mate Lane would be chosen as acting president by the Senate) most Republicans would still hope for a victory in 1864, and favor remaining in the Union. Or at least enough of them would do so, that combined with Democrats, they could block northern secession.
In short, a fair number of antislavery northerners did toy, at least rhetorically, with disunionism--but "toyed" and "rhetorically" are the key words here. (And even in the unlikely event it occurs, would a secession limited to, say, the "fanatical" New England states necessarily lead to a civil war? I could see a lot of Southerners and doughfaces who would be happy with the weakening of the Republican Party's national prospects this would entail, and "let the erring sisters depart" in peace...)