The political party that was established to oppose the extension of slavery into western territories was the Republican Party.
The Republican Party was founded in 1854, primarily as an anti-slavery party. Its formation was a response to the passage of the Kansas-Nebraska Act, which allowed for the possibility of slavery expanding into territories that had been designated as free under the Missouri Compromise of 1820.
The Republican Party's first presidential candidate, John C. Fremont, ran on an anti-slavery platform in the 1856 election.
The party's opposition to the expansion of slavery played a significant role in the lead-up to the American Civil War.