Actually I'm only aware of two Democratic conventions that year, the first in Charleston that had to be suspended and the second one resuming later in Baltimore. Are you counting the expulsion of the Southern delegates at the latter site as a third convention or did I miss one?
Yes! The expulsion, and then those delegates went to another site and had their own convention! Boy, people sure knew how to have fun in those days. God forbid we have so much fun as that anytime soon.
The Whigs had already collapsed largely due to their inability to come to an agreement on what stance to take on Slavery, which opened the door for the Republicans, but an offshoot of the Whigs (Constitutional Unionists) did run a significant showing in 1860; The CUs were for keeping the union intact while also keeping Slavery from spreading; its candidate John Bell won his home state of Tennessee, Lincoln's and Breckinridge's home state of Kentucky, and Virginia -- all "border" states. Another "border" state, Missouri, was the only one the Democrat Douglas won although he did get a split EV from New Jersey which split between Lincoln and Douglas, so at least that state wasn't doing WTA at the time.
Wonderful details. I actually knew the Whigs and Republicans switched around --- I'm going to say the previous election, 1856, IIRC, but it was part of the whole soup even later, as the Constitutional Unionists showed in 1860.