What group was strongly determined to stop the spread of slavery into the Western territories.?

The group strongly determined to stop the spread of slavery into the Western territories was the Free Soil Party. Founded in 1848, this political party advocated for the prevention of slavery's expansion into newly acquired lands following the Mexican-American War. They believed that free men on free soil constituted a morally and economically superior system to slavery. Their stance was a significant factor in the growing sectional tensions leading up to the Civil War.