But the advocates of slavery have affirmed a strange doctrine in regard to the Constitution. They think that because I swore to support the Constitution, I swore to support the practice of slaveholding. Sir, slaveholding in Virginia is no more under the control or guarantee of the Constitution than slavery in Cuba, or Brazil, or any other part of the world is under the control or guarantee of the Constitution. Not one principle.

As quoted in His Brother's Blood: Speeches and Writings, 1838–64 (2004), edited by William Frederick Moore and Jane Ann Moore, p. 199 - Speech to the U.S. House of Representatives (April 1860)