What was the "smoking gun"? Why did it lead most of President Nixon's remaining supporters to abandon him?

While Nixon battled against the Watergate investigators, his vice president, Spiro T. Agnew was forced to resign his office on October 10, 1973, amid charges of financial corruption when he had been governor of Maryland. While unconnected to the Watergate affair, Agnew's resignation added to the appearance of scandal hanging over the Nixon administration. Nixon appointed Gerald R. Ford, a longtime Michigan Congressman, as vice-president, and Ford was almost immediately confirmed by the House and the Senate. Many Americans, including the president, knew that Ford might well become president if the Watergate scandal continued to engulf Nixon.

In the summer of 1974, the House Judiciary Committee recommended that the president be impeached for abusing the powers of his office and attempting to obstruct Congress's investigation of Watergate. The U.S. Constitution gives the House of Representatives the responsibility to impeach (indict) the president and other government officials who have committed "high crimes or misdemeanors." A president impeached by a majority vote in the House is then tried in the Senate, where a two-thirds vote is required to remove the president from office. Only one president, Andrew Johnson, had previously been impeached. None has ever been convicted and removed from office.

Nixon's last-ditch efforts to retain the presidency failed on July 24, 1974, when the U.S. Supreme Court ruled unanimously in the case of U.S. v. Nixon that the president must surrender the tape recordings subpoenaed by the special prosecutor. The conversations on these tapes revealed that the president had been fully aware of the burglary within days of its occurrence. They also revealed that Nixon himself had orchestrated much of the cover-up and that he had personally ordered the FBI to stop investigating the matter. The tapes also revealed that Nixon was a crude leader, given to profanity and mean-spiritedness. These tapes supplied the so-called "smoking gun," directly linking the president to the cover-up. In light of this evidence, all but his most devoted supporters in the House and Senate abandoned him.