As recounted in Douglas Egerton’s excellent book on the 1860 election, The Year of Meteors, the big issue in the election of 1860 was extension of slavery to the new territories. The Democratic Party met in Charleston SC on April 23, 1860 to nominate its candidate, with Sen. Stephen Douglas (D-Ill) as the frontrunner. After the convention refused to adopt a pro-slavery extension platform, southern states, lead by “fire eaters” like William Yancey of Alabama walked out and convened their own convention. The Party had adopted a 2/3 needed for nomination rule, and back at the original convention Douglas led on all 57 ballots, but never got the 2/3 majority, and the convention adjourned on May 3.
On June 11, some southern delegates convened in Richmond, but only a South Carolina, Florida and a few delegates from other southern states showed up, and they decided to see what the rest of the party would do.
The Democrats reconvened in Baltimore on June 18, but on the fifth day, after a credentials dispute concerning new southern delegations, southern delegates walked out again. The convention nominated Douglas, with 2/3 of the votes cast. Southern delegations convened at another location in Baltimore and nominated as president Vice President John Breckenridge (who later served as a major general in the Confederate army). Egerton notes that neither Douglas nor Breckendridge received the required 2/3 of the elected delegates and thus were not properly nominated.
On June 26, the Richmond convention met again and endorsed Breckenridge.
The Republican Party, meeting in Chicago on May 18 nominated Abraham Lincoln (over favorite William Seward), and a new Constitutional Party (favoring compromise and Union) nominated John Bell of Tennessee on May 9 in Baltimore. In the general election, Virginia went for Bell, but Lincoln won 1ith 180 of 303 possible electoral votes. Egerton convincingly argues that the militant southerners had determined that slavery would never be safe in the Union and thus intended to split the Democratic Party and elect an arguably anti-slavery Republican as an excuse for succession. They succeeded, but of course, ultimately, the Union was preserved and slavery was abolished.