Tilt the 1860 election enough that the electoral college is hung. There's probably enough of a possibility of a deadlock in the House that Breckenridge could become Acting President in March which might be enough for the North to secede.
How the heck are you going to do that?
As pointed out in the thread
https://www.alternatehistory.com/discussion/showthread.php?t=333109
And who's to say that a unified Democratic ticket would still be able to defeat Lincoln?
Remember, even if you combined ALL of the non-Lincoln votes together into one hypothetical candidate, Lincoln STILL would have won despite winning less than 40% of the popular vote against a "candidate" that would have won over 60%. That kinda blows your mind but it's mathematically true...Lincoln basically campaigned in only half of the country, and his vote totals showed that. It was the electoral college that saved him in the end, and even a unified Democratic ticket probably wouldn't have been able to defeat him.
(post by 1.36)
OTL, even if the Democrats had united under a single candidate and said single candidate got every vote that each of OTL's three got, Lincoln would still win.
So. You have to make massive massive changes for the election to go to the House. You'd have to have a radical fire eater as the Republican candidate AND you'd have to get the Democrats more united.
And why do you say Breckinridge would be come acting president? He was a presidential candidate, not a vice-presidential one.
And the vice-presidential vote in the Senate is a straight up/down choice between the top two candidates. So one would surely win. No?