How did the Kansas-Nebraska Act change the earlier Missouri Compromise?

The Kansas-Nebraska Act of 1854 fundamentally altered the Missouri Compromise of 1820 by effectively repealing it.12

Under the 1820 compromise, Congress had prohibited slavery in the Louisiana Purchase territories located north of the 36°30′ parallel. The Kansas-Nebraska Act, sponsored by Senator Stephen A. Douglas, replaced this congressional restriction with the principle of popular sovereignty. This doctrine allowed the settlers within the Kansas and Nebraska territories to decide for themselves whether to permit slavery, rather than adhering to the geographic boundary previously established by federal law.312

By removing the established prohibition on slavery in these northern territories, the act intensified sectional tensions, contributed to the rise of the Republican Party, and led to the violent conflict known as Bleeding Kansas.32