David M. Potter describes “Free Soilers” as “antislavery dissenters within both old parties.”(1)
Michael F. Holt provides an excellent profile of the Northerners who made up the “Free Soil” Party in his description of the delegates, both elected and self-appointed, who attended the first convention of the party in Buffalo.
“Uniformly zealous, they were a heterogeneous lot: Midwestern Democratic proponents of rivers and harbors improvements, which neither party had officially endorsed and Polk had vetoed; labor reformers interested in free homesteads in the West; and even vengeful Clay loyalists from New York City. But most were primarily determined to stop slavery’s spread, and they included three main groups: antislavery Whigs from New England and the Midwest; antislavery Democrats, including New York’s Barnburners; and Liberty men.”(2)
Holt goes on to reinforce that “by itself, antislavery sentiment does not explain who became Free Soilers and who did not.” (3) It varied by state and had much to do with how eager statesmen were to stay with trusted leaders from older existing parties.
(1) David M. Potter, “The Impending Crisis 1848 – 1861,” (New York: Harper Perennial, 1976), 227.
(2) Michael F. Holt, “The Rise and Fall of the American Whig Party: Jacksonian Politics and the Onset of the Civil War,” (New York: The Oxford University Press, 1999), 339-340.