Final answer:
The Kansas-Nebraska Act, passed in 1854, allowed for the expansion of slavery into the territories of Kansas and Nebraska. This act had major political consequences, leading to the formation of the Republican Party.
Step-by-step explanation:
The Kansas-Nebraska Act, passed by Congress in 1854, was an important event leading to the Civil War. It divided the territories of Kansas and Nebraska and allowed for the expansion of slavery into these territories through the principle of popular sovereignty. This act undermined the Whig party and led to the formation of the Republican Party, which attracted northern Whigs, anti-Kansas-Nebraska Act Democrats, Free-Soil Party members, and abolitionists.