Wendell Phillips
Speech to the Massachusetts Anti-Slavery Society, 28 January 1852
Phillips used it in a call for abolitionists to keep up the pressure and not to feel secure in small wins.
the myth (Thomas Jefferson): Jefferson is often credited, but the phrase is not in his writings. The idea goes back to an Irish judge, John Philpot Curran, in 1790, and the short version belongs to the abolitionist Phillips.
Source: Wikiquote