The "Daisy" ad, of course.
Did you know that the DNC paid to run the ad only once!
Yet, virtually everybody knew about it, and most had actually seen it.
Because the networks kept showing it and commenting on it in their news shows...over-and-over.
Even in 1964, the media knew which side they were on.
See Walter Duranty's NY Times reports from the Soviet Union in the early 1930s.