Why did the expansion of slavery become the most divisive political issue in the 1840s and 1850s?

The expansion of slavery became the most divisive political issue in the 1840s and 1850s due to the conflicting interests of free and slave states as the United States expanded westward. The debate centered on whether new territories and states admitted to the Union would be free or slave-holding, which threatened to upset the delicate balance of power between North and South. This conflict intensified with events such as the Missouri Compromise, the Compromise of 1850, and the Kansas-Nebraska Act, leading to violent confrontations like "Bleeding Kansas" and fueling sectional tensions that ultimately culminated in the Civil War.