Sir, than robbery, than piracy, than polygamy, slaveholding is worse. More criminal, more injurious to man, and consequently more offensive to God. Slaveholding has been justly designated as the sum of all villainy. Put every crime perpetuated among men into a moral crucible, and dissolve and combine them all, and the resultant amalgam is slaveholding. It has the violence of robbery.

As quoted in His Brother's Blood: Speeches and Writings, 1838–64 (2004), edited by William Frederick Moore and Jane Ann Moore, pp. 192–193 - Speech to the U.S. House of Representatives (April 1860)