Richard Nixon actually said...
Well, I think first, Mr. Smith, with regard to what you call the credibility gap, that many observers--in fact, I think I even recall something you said at one time--have pointed out that Presidents, particularly when they have difficult problems in foreign affairs, inevitably are going to acquire some credibility gap.
Context
A Conversation With the President, Interview With Howard K. Smith of the American Broadcasting Company
03/21/1971