Final answer:
Stephen A. Douglas proposed the Kansas-Nebraska Act which led to the repeal of the Missouri Compromise and opened the door to slavery in new territories through popular sovereignty. This act provoked a national controversy and restructured the political parties of the time.
Step-by-step explanation:
Douglas advocated for the repeal of the Missouri Compromise, leading to the creation of two new territories: Kansas and Nebraska. This legislative action was part of what is known as the Kansas-Nebraska Act, a bill introduced by Stephen A. Douglas. Despite opposition, the act was approved by Congress and it had significant consequences, including allowing the possibility of slavery in these new territories through the principle of popular sovereignty. The Kansas-Nebraska Act upset the fragile balance between slave and free states, and directly influenced heightened sectional tensions that preceded the Civil War. It also contributed to the collapse of the Second Party System and the rise of the Know-Nothings and the Republicans as replacements for the Whigs, changing the political landscape of the period.