In Nixon's case, it was tapes he kept of conversations from the Oval Office; in Mr Clinton's case, the tapes secretly recorded by Linda Tripp and the videotape of the President's testimony to the grand jury.
In the Nixon case, Congress fought up to the Supreme Court to gain access to the original tapes, refusing along the way to be satisfied with transcripts.
The surrender of the Nixon tapes, which was ordered on 28 July 1974, is seen by historians as a turning point, not only in the decision of Congress to impeach the President, but also with public opinion, which had hitherto preferred to keep his difficulties at arms' length.
For Congress, the key was the tape of 23 June, 1972, which proved that Mr Nixon had tried to involve the CIA against the FBI in the cover-up of the Watergate break-in.
With public opinion, however, it was the unexpectedly coarse tone of the President's discourse, plus the fact that the transcripts were peppered with the infamous phrase, "expletive deleted", that is credited with sending Mr Nixon's support plummeting and convincing him that he could not survive in office. His resignation took effect on 9 August, 1974.Reuse content