Lincoln was propelled to candidacy by virtue of his strong performance in the Lincoln-Douglas debates and his extraordinary speech at Cooper Union in New York in 1860 where he urged the crowd to have faith that "right makes might." His nomination was still an upset. As a candidate, he opposed the expansion of slavery.
Douglas tried to be the only national candidate in the race but could not unify his party. His nomination was opposed by the sitting president, James Buchanan. He continued to support popular sovereignty. Southern Democrats insisted on a federal slave code allowing slaves in a territory. The territory could only vote out slavery right before it was to become a state. Douglas refused.
The Constitutional Union party was made up of people who did not feel at home in either party. The party blamed both the Republicans and Democrats for the sectional divide and ran simply on upholding the law. Bell, a former Senator, took no position on slavery and simply hoped people would rally to his party to avoid war.
Breckinridge, the sitting Vice President, was the leading candidate from the South. He endorsed popular sovereignty with the federal slave code for the territory that Douglas denied. He hoped that his candidacy could force the election to the House of Representatives.