Final answer:
Supporters of slavery favored the Kansas-Nebraska Act as it allowed for the expansion of slavery into new territories via popular sovereignty, undermining the Missouri Compromise and potentially increasing pro-slavery political power.
Step-by-step explanation:
The people who wanted to keep slavery generally supported the Kansas-Nebraska Act because it offered them the possibility of expanding slavery into new territories. The Act was introduced in 1854 by Stephen A. Douglas and sought to organize the Kansas and Nebraska territories, significantly by opening them to slavery which had previously been restricted by the Missouri Compromise. The underlying principle of the act was popular sovereignty, allowing the settlers of the territories to decide the issue of slavery by vote, which appealed to Southern interests.
This action essentially repealed the Missouri Compromise line, and many Southerners saw it as an opportunity to spread slavery into the West and gain political power, as new slave states would bolster their representation in Congress. However, the prospect of extending slavery encouraged a rush of both pro-slavery and anti-slavery settlers into Kansas, leading to conflicts known as Bleeding Kansas, as violent confrontations erupted over the control of the state's future.