Answer:
Popular sovereignty
Step-by-step explanation:
The South Democrats (who were vehemently pro-slavery) rejected Douglas's nomination as president and abandoned the party convention. They decided to name their own candidate, then Vice President John C. Breckinridge. The reason they rejected Douglas was because they did not support Douglas's concept of popular sovereignty.
A State in which People's Sovereignty rules is created and subject to the will of the people, who are the source of all political power.