The Kansas-Nebraska Act was introduced in 1854 by Illinois Democrat Stephen Douglas to create the territories of Kansas and Nebraska. Within, it also introduced the idea of “popular sovereignty,” in which residents of Kansas Territory would vote on whether or not to enter the Union as free-states or slave-states. Its enactment effectively repealed the Missouri Compromise of 1820, which outlawed slavery north of the 36º30' parallel north (Missouri’s southern border). In a United States already locked in debate over enslavement, the Kansas-Nebraska Act of 1854 further aggravated tensions.  

Section 19 of the Kansas-Nebraska Act outlines this, with the most relevant portion italicized: 

And be it further enacted, That all that part of the Territory of the United States included within the following limits, except such portions thereof as are hereinafter expressly exempted from the operations of this act, to wit, beginning at a point on the western boundary of the State of Missouri, where the thirty-seventh parallel of north latitude crosses the same; thence west on said parallel to the eastern boundary of New Mexico; thence north on said boundary to latitude thirty-eight; thence following said boundary westward to the east boundary of the Territory of Utah, on the summit of the Rocky Mountains; thence northward on said summit to the fortieth parallel of latitude, thence east on said parallel to the western boundary of the State of Missouri; thence south with the western boundary of said State to the place of beginning, be, and the same is hereby, created into a temporary government by the name of the Territory of Kansas; and when admitted as a State or States, the said Territory, or any portion of the same, shall be received into the Union with or without slavery, as their Constitution may prescribe at the time of their admission: Provided, That nothing in this act contained shall be construed to inhibit the government of the United States from dividing said Territory into two or more Territories, in such manner and at such times as Congress shall deem convenient and proper, or from attaching any portion of said Territory to any other State or Territory of the United States: Provided further, That nothing in this act contained shall be construed to impair the rights of person or property now pertaining to the Indians in said Territory, so long as such rights shall remain unextinguished by treaty between the United States and such Indians, or to include any territory which, by treaty with any Indian tribe, is not, without the consent of said tribe, to be included within the territorial limits or jurisdiction of any State or Territory; but all such territory shall be excepted out of the boundaries, and constitute no part of the Territory of Kansas, until said tribe shall signify their assent to the President of the United States to be included within the said Territory of Kansas, or to affect the authority of the government of the United States to make any regulation respecting such Indians, their lands, property, or other rights, by treaty, law, or otherwise, which it would have been competent to the government to make if this act had never passed.

Enacting the Kansas-Nebraska Act led to Bleeding Kansas, a series of violent confrontations that killed dozens. These series of events pushed the United States closer to Civil War, which broke out in 1861. When the war eventually began, Stephen Douglas supported the Union; he died before the War ended.