To argue the point, editorials in Southern newspapers frequently discussed ways and means of adding new slave states. During 1856-7, Southerners organized expeditions to Kansas in an effort to bring that state in the union as a slave state. I would call this pro-slave activism. The same holds true for the William Walker expedition to Nicaragua, another possible slave state. Clearly Southerners had some interest in maintaining a semblance of parity in the U.S. Senate.