The smoking gun tape refers to a crucial audio recording made on June 20, 1972, that captured President Richard Nixon discussing the Watergate cover-up with his chief of staff, H.R. Haldeman. This tape served as direct evidence of Nixon's involvement in the scandal, solidifying suspicions about his administration's attempts to obstruct justice and mislead investigators. The tape became pivotal in the investigation and eventual resignation of Nixon, symbolizing the depths of corruption in his presidency.