What Event made slavery a central political issue in the mid-1850s?

The passage of the Kansas-Nebraska Act in 1854 made slavery a central political issue in the mid-1850s by allowing the territories of Kansas and Nebraska to decide for themselves whether to allow slavery through popular sovereignty. This led to violent conflicts known as "Bleeding Kansas" as pro-slavery and anti-slavery factions clashed. The act also effectively repealed the Missouri Compromise, heightening tensions between free and slave states and polarizing national politics around the issue of slavery.