Prehapse get a repeat of 1824 where you have a guy who does not win the electoral college, populer vote or even number of states still become president due to the syestem breaking down do to more then 2 populer candidates (a 4 way split caused our electoral process to grind to a halt in 1824 and a repeat could be the cival war fual your looking for)
en.m.wikipedia.org
Seeing as you once again have a 4 way election in 1860 and almost the exact same populer vote % between Lincoln and Douglass that Lincoln and adams had, and Breckinridge a ok spread of states in 1860 I dont think it's out of the question that it could happen again if you had a diffrent list of candidates or if existing ones campaigned differently or formed joint tickets. (If Breckinridge [southern democrat] and Douglas [one of Lincoln great rivals having most recently beat Lincoln in the 1958 Illinois senate race] can reconcial thier diffrances and form a joint democratic ticket in united in opposition of Lincoln even if they did not see eye to eye on slavery it looks like they might be able to beat Lincoln in the populer vote but still conceivably lose the electoral vote)
Becuse we have a 4 year window prior prehapse effect the Lincoln Douglass and damage Lincoln before he takes the national stage to campaigne in the 1860 election. Or stire up more bad blood between the two by makeing thier prior senate races get ugly (mud slighing, bad faith arguments, personal attacks, ect) so Douglass is more willing to sign up for a anti Lincoln ticket.
en.m.wikipedia.org
en.m.wikipedia.org
en.m.wikipedia.org