Final answer:
The Kansas-Nebraska Act of 1854 allowed new territories to decide on the issue of slavery, leading to political upheaval and contributing to the Civil War.
Step-by-step explanation:
The Kansas-Nebraska Act of 1854 was a significant piece of congressional legislation that affected the power balance between the federal government and the states. Introduced by Illinois Senator Stephen A. Douglas, the Act created the territories of Kansas and Nebraska and allowed the settlers in those territories to decide by popular sovereignty whether they would allow slavery or not. This effectively overruled the previous Missouri Compromise which had prohibited slavery in that region.
After passage of the Act, the focus was particularly on Kansas, where the future status of slavery hung in balance. Confrontation in Kansas escalated as pro-slavery and anti-slavery settlers moved there to influence the state constitution, leading to a period of violence known as "Bleeding Kansas." The implications of the Kansas-Nebraska Act deeply divided Democrats, contributing to the split of the party along sectional lines and setting the stage for more profound conflicts over slavery.