Nixon & Watergate, from "The American President" website. I quote:

"In late June 1973, the roof fell in when White House Counsel John Dean testified before the Senate committee investigating Watergate. He claimed there was "a cancer growing on the presidency" and implicated Nixon himself in the cover-up of the Watergate affair. A month later, another White House aide, Alexander Butterfield, revealed that the president had covertly tape-recorded all his conversations in the Oval Office. The evidence was subpoenaed by a special prosecutor investigating Watergate and by the Senate committee.

(In an unrelated investigation, massive financial improprieties were traced to Vice President Spiro Agnew, including tax evasion and outright bribes while he had been governor of Maryland. He resigned his office on October 10, 1973 and pled nolo contendre (not contesting charges) in federal court. Nixon nominated the House Republican Minority Leader Gerald R. Ford for the vice presidency. Under provisions of the Twenty-fifth Amendment, Ford was confirmed by both the House of Representatives and the Senate.)

From this point forward, the great political battle turned on the tapes. Nixon claimed that "executive-privilege" made him exempt from legal subpoenas ordering him to hand them over to investigators. The special prosecutor, Archibald Cox, pressed Nixon for the tapes—and Nixon asked the attorney general to fire Cox. Attorney General Eliott Richardson refused, claiming he had promised the Senate that he would protect the independence of the investigation. Richardson, resigned in protest rather than carry out the order. Nixon then asked Deputy Attorney General William Ruckelshaus, to fire Cox. Ruckelshaus refused and Nixon fired him. Finally, Solicitor General Robert Bork agreed to fire Cox. This episode, known as the "Saturday Night Massacre," seemed to many Americans to resemble a coup d'etat against the rule of law. Nixon buckled, and two days later promised to release nine tapes that Cox had demanded. He also had to appoint a new special prosecutor, Leon Jaworski, who proved just as tenacious in going after evidence.

Nixon had lost public confidence. By March of 1974 his two top aides and former Attorney General Mitchell were indicted. Nixon stubbornly refused to surrender all the tapes. A federal court ruled that he did not have to turn them over to the Senate committee, but that he did have to provide them to the federal grand jury and special prosecutor Jaworski. In July, the Supreme Court ruled unanimously in U.S. v. Nixon against his claim of executive-privilege and ordered him to turn over the tapes. Meanwhile, on July 27, the House Committee on the Judiciary approved three articles of impeachment against him involving obstruction of justice and the abuse of presidential power. "Richard M. Nixon, using the powers of his high office," the first article concluded, "engaged personally and through his subordinates and agents in a course of conduct or plan designed to delay, impede and obstruct investigations . . . to cover-up, conceal and protect those responsible and to conceal the existence and scope of other unlawful activities." The second article charged Nixon with using the CIA, FBI, Secret Service and IRS to harass opponents of the administration. It also charged him with maintaining "a secret investigative unit within the office of the President" that "engaged in covert and unlawful activities." The third accused him of obstruction of justice for refusing to cooperate with Congress in the inquiry. All of the articles of impeachment were approved by the Democrats and a small group of Republicans on the committee."

http://www.americanpresident.org/kotrain/courses/RN/RN_Domestic_Affairs.htm