How did slavery cause sectionalism in the antebellum era?

Southern politicians supported the expansion of slavery into new states.

Explanation:

Antebellum is a Latin word meaning "before the war". In the United States the term antebellum refers to the period in which secessionism was increased by the Confederate States of America, which led to the American Civil War between the States of the Union and the Confederates.

In this context, the two main causes of secession were the Republican intention of restricting, even eliminating, slavery and the great limitations of political power held by the states of the Union, especially with regard to the ownership of slaves

The white Southerners believed they had the right and obligation to maintain slavery for economic and social self-protection and supported the expansion of slavery into new states.