Who was party opposed to expanding slavery?

The party opposed to expanding slavery was primarily the Republican Party, which was founded in the 1850s. It emerged in response to the Kansas-Nebraska Act and the growing tensions over the spread of slavery into new territories. Key figures in the party, such as Abraham Lincoln, advocated for limiting slavery's expansion as part of a broader platform promoting free labor and economic opportunity. This opposition ultimately contributed to the sectional conflicts leading up to the American Civil War.