The Barnburner faction was a group of New York Democrats who opposed the southern (pro-slavery) wing of the Democratic Party.
The nickname came from a story about a Dutch farmer who burned his barn in order to get rid of the rats. The New York Barnburners advocated the Wilmot Proviso that intended to block slavery in territory acquired from Mexico following the Mexican-American War. In the 1848 presidential election, the Barnburners left the Democratic Party after it nominated Lewis Cass and were instrumental in organizing the third-party Free Soil Party and nominating Martin Van Buren. With the split in the Democratic Party, Cass went down to defeat. Following 1848, most of the Barnburners migrated towards the Republican Party (formed in 1854).
Potter, David M. The Impending Crisis, 1848–1861. Edited and completed by Don E. Fehrenbacher. New York: Harper and Row, 1976.
Rayback, Joseph G. Free Soil: The Election of 1848. Lexington, Ky.: University Press of Kentucky, 1970.
Shearer, Augustus H. "Barnburners." Dictionary of American History. Ed. Stanley I. Kutler. 3rd ed. Vol. 1. New York: Charles Scribner's Sons, 2003. 417-418.