Source, in full
At a Republican primary debate in Nashua, New Hampshire (23 February 1980). The moderator had ordered the soundman to turn off Reagan's microphone, as Reagan was about to explain that as his campaign was paying for the debate which had originally been arranged by the Nashua Telegraph and that he had invited Bob Dole, Howard Baker, John B. Anderson, and Phil Crane, to be in it as well.
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Video footage