Answer:
Kansas-Nebraska Act
Explanation:
The United States had already acquired a <em>VAST</em> amount of sparsely settled land in the 1803 Louisiana Purchase, but ever since the 1840s Stephen A. Douglas (first picture below) had sought to establish a territorial government¹ in a portion of the Louisiana Purchase that was still unorganized. Douglas's efforts were stymied² by Senator David Rice Atchison (second image below) and other Southern leaders who refused to allow the creation of territories that banned slavery, which would have been banned because the Missouri Compromise would not allow any territories north of latitude 36°30' north to have slavery. So, to win Southern support, Franklin Pierce (third picture below) and Douglas eventually agreed to back the repeal of the Missouri Compromise, with the status of slavery to instead be decided by popular sovereignty.
Douglas's bill would (surprisingly) win approval by a wide margin in the Senate (37 aye - 14 nay), but faced stronger opposition in the House of Representatives. (113 aye - 100 nay). Northern Whigs strongly opposed the bill, but that wasn't enough to to prevent it from being passed. After the passage of that act, pro-and anti-slavery elements would flood into Kansas to establish a population that would either vote for or against slavery, which resulted in a series of armed conflicts known as "Bleeding Kansas." (or Bloody Kansas or Border War) Douglas and Pierce hoped that popular sovereignty would help bring an end to the national debate over slavery, but that went wide right. The Kansas–Nebraska Act outraged many Northerners. This gave rise to the anti-slavery Republican Party, which debuted in 1858. Ongoing tensions over slavery would eventually lead to the American Civil War to begin in 1861.
¹territorial government - An administrative body/system in which political direction or control is exercised over a designated area/an administrative division of a city/county/larger geographical area
²stymied - prevent or hinder the progress of
