Question

Which would most likely have been supported by states’ rights advocates during the 1860 presidential election?

Answer 1

They would have supported the idea of popular sovereignty. 

Answer 2

States' rights advocates would have supported John C. Breckinridge during the 1860 presidential election.

Breckinridge's electoral program was almost identical to that of the other three candidates (especially in terms of increasing Federal Government spending on infrastructure), except for two major differences; one of those differences was that Breckinridge did not want to raise tariffs as the other candidates wanted to do. But the main difference was in the issue of the slavery of African-Americans, because Breckinridge proposed extending slavery to the Western Territories that were on the way to becoming States, while the others opposed this measure (and in the case of Lincoln he even wanted to eliminate it some day in the Southern States).

The Democrats of the South and their candidate Breckinridge wanted to legalize slavery in those territories so that when they were converted into States and therefore they will begin to send Senators and Representatives to Congress and participate in the presidential elections; they could tip the balance of political power in favor of the slavers and thus ensure that the institution of slavery was never eliminated by the abolitionists of the North.