Stephen A. Douglas, Illinois Democratic senator.
Like many northern Democrats by the 1850s, he had to serve the slave power to survive. Ultimately, his willingness to compromise destroyed him. Although he beat Lincoln in the Senate race in 1858, he lost in 1860, primarily because the southern Democrats walked out of the convention when Douglas was nominated. This despite the fact that Douglas did everything he could to destroy the Missouri Compromise through the Kansas-Nebraska Act of 1854, which introduced the concept of "popular sovereignty," which supposedly would let people decide as a state whether they wanted slavery. What this really meant was that pro and anti-slavery zealots flooded to Kansas to start killing each other over the issue in 1857. So-called "Bleeding Kansas" was a prelude to the Civil War and few deserved more blame for it than Douglas. Despite this, he wasn't considered ideologically pure enough for the Democratic Party of the time.
Any comparisons between the Democratic Party of 1860 and the Republican Party of 2009 are up to the reader to decide.