I would remind you that extremism in the defense of liberty is no vice. And let me remind you also that moderation in the pursuit of justice is no virtue.
Barry Goldwater (1909-1998) American politician
Speech, accepting the GOP Presidential Nomination, San Francisco (16 Jul 1964)
(Source)
Goldwater believed the phrase originated in Cicero, though the source he used is questionable. Karl Hess was Goldwater's speech writer, and he said he derived the turn of phrase from Lincoln's "House Divided" speech. A closer match is this Thomas Paine passage.
More discussion of this quotation and its origins: On the Saying that "Extremism in Defense of Liberty is No Vice" - Niskanen Center
