Answer: The Kansas-Nebraska Act.
Step-by-step explanation:
The Kansas-Nebraska Act was the law enacted in the United States, in 1854, for the creation of the states of Nebraska and Kansas, in territories of former French Louisiana.
The situation of the two states north of the line defined in the Missouri Compromise meant that both should be states in which slavery was not allowed. However, the contiguity of Kansas with the slave state of Missouri and the search by Senator Douglas for southern support for a railroad in his state (Illinois) caused the law to include the provision that, in order to decide on the issue of slaves, citizens could exercise "popular sovereignty" and, therefore, be able to decide whether to be a slave state or not.
The discussion of the law and subsequent voting provoked strong conflicts between anti-slavery and pro-slavery, especially in Kansas, and the disappearance of the Whig party (divided between supporters of the law in the south and those opposed to it in the north), and the creation of the Republican Party. To the new party were incorporated, in addition to the most determined anti-slavery, those who opposed the expansion of slavery, although accepting it in a certain way, limiting its existence to the states where it already existed. That position against slavery, although not abolitionist, allowed the Republican Party to be the dominant force in the north, and not lose all the southern vote, and that its candidate,Therefore Abraham Lincoln, won the presidential election in 1860.