Hopefully, you've heard to story of how Charles Sumner was beaten almost to death on the Senate floor in 1856. Sumner had delivered an incendiary anti-slavery speech a couple of days earlier in which he savagely abused his assailant's cousin, Sen. Andrew Butler of South Carolina. When Rep. Preston Brooks entered the Senate with his cane, he was looking to protect the honor of his family and his state. What you might not know is that both Brooks and Sumner were Democrats at the time.
In truth, I am not certain how to characterize Sumner's political affiliation on May 22, 1856. He had been elected to the Senate in 1850 as a Free Soil Democrat, but that political movement had effectively collapsed after the 1852 presidential election. The Massachusetts General Court gave him another six-year term in November 1856, “believing that his vacant chair in the Senate chamber served as a powerful symbol of free speech and resistance to slavery,” but his injuries were so severe that he didn't return to duty in the Senate until 1859. By that time, he was clearly a member of the Radical Republican faction of a new political party.