Which political party formed in opposition to the expansion of slavery after the Kansas-Nebraska Act?

The formation of the Republican Party in opposition to the expansion of slavery was a significant reaction to the political climate created by the Kansas-Nebraska Act of 1854. This act allowed for the possibility of slavery in territories that had previously been designated as free, which intensified the national debate over the issue. The Republican Party was established primarily by former Free Soilers, anti-slavery Democrats, and former Whigs who were united in their commitment to oppose the extension of slavery into the western territories.

The party's platform centered on the idea of stopping the spread of slavery, appealing to those who were concerned about the moral implications of slavery as well as its political and economic consequences. The Republican Party's emergence marked a pivotal moment in American history, leading to the realignment of political factions and contributing to the onset of the Civil War.

The other political parties mentioned did exist during this period, but they did not specifically arise in direct opposition to the expansion of slavery like the Republican Party did. Therefore, the establishment and political goals of the Republican Party make it the correct answer to this question.