Tracking a phrase: "Speak truth to power"

A commenter on this post at The Monkey Cage discusses the phrase "speak truth to power," which, according to the commenter, stems from "a Quaker assertion of the eighteenth century" which was echoed in an American Friends Service Committee publication from the 1950s. The commenter also notes that the phrase was used by political scientist Aaron Wildavsky in the title of a 1979 book.

Another use of the phrase, one that predates Wildavsky, was in Hans J. Morgenthau's collection Truth and Power: Essays of a Decade, 1960-70 (Praeger, 1970). Morgenthau dedicated the book to one of his mentors: "to Hans Kelsen, who has taught us through his example how to speak Truth to Power." And in the prologue Morgenthau wrote:

"In the long run..., the voice of truth, so vulnerable to power, has proved more resilient than power. It has built empires of the mind and the spirit that have outlasted, and put their mark upon, the empires of power. On January 22, 1967, about thirty people demonstrated in Pushkin Square in Moscow against the arrest of four persons who had transcribed the court records in the trial against Andrei Sinyavsky and Yuri Daniel. One of the organizers of the protest, Khaustov, who was sentenced to three years at hard labor, admitted at his trial that he had read Kant and Hegel and that his reading of Kant 'made me see a lot of things in a new light.' The experience of the 1960's has dispelled the illusion that truth can show power the way in direct confrontation. But historical experience reassures us that truth can indeed make people 'see a lot of things in a new light.' And when people see things in a new light, they might act in a new way."

Not exactly the side of Hans Morgenthau that most students get in their introductory international relations classes, is it?