Final answer:
The Kansas-Nebraska Act upended the second-party system and increased the membership of the Republican Party dedicated to popular sovereignty.
Step-by-step explanation:
Kansas-Nebraska Act:
The Kansas-Nebraska Act was one of several events that upended the second-party system of Whigs and Democrats and increased the membership of the Republican Party, which dedicated itself to the principles of popular sovereignty.
The Kansas-Nebraska Act was introduced by Stephen A. Douglas and allowed the people of the territories to decide whether they would allow slavery through popular sovereignty. This act went against the Missouri Compromise which had prohibited slavery in these territories. The passage of the act led to increased support for the Republican Party, as people opposed to the extension of slavery united under its banner.