Why did Jefferson believe that the passage of the Missouri compromise would deepen sectional conflict

its slavery

Jefferson believed that the passage of the Missouri Compromise (agreement involving regulation of slavery in western territories) would deepen the sectional conflict because he believed that the argument over the line would last forever between Americans.

Sectionalism is the different economies, social structure, customs, and political values of the North and the South and he believed that the Compromise would lead to the destruction of the Union.