The visit to Peking took place in February and he was invited to meet Chairman Mao Zedong, a mark of high respect. In May, he visited Moscow and signed the agreement limiting the nuclear arsenals of the United States and the Soviet Union. In the presidential election of 1972 Nixon and Agnew ran against Democrats George McGovern and Sargent Shriver. The election was a landslide for Nixon, as the polls had predicted it would be: he won 61 percent of the popular vote and received 521 electoral votes, losing only Massachusetts and the District of Columbia. However, as in the election of 1968, the Democrats retained control of Congress. During his last election campaign, what first appeared as a minor burglary was to become the beginning of the end of Nixon's political career. A break-in at Democratic national headquarters in Washington, D.C.'s Watergate apartment complex was linked to Republicans. During the trial of six men charged in the crime, the existence of the cover-up began to emerge, taking government officials down like dominos in its path. Nixon elicited the resignation of two top aides in April 1973 in an effort to stem the tide. But in October, as the Watergate investigation continued, he lost his vice president, Spiro T. Agnew, who resigned before pleading "nolo contendere" (no contest) in federal charges of income tax evasion related to accusations of accepting bribes. Nixon's efforts to avoid the taint of those scandals were fruitless when subpoenaed tapes he was ordered to give up by the U.S. Supreme Court showed he obstructed justice in stopping an FBI probe of the Watergate burglary. On August 9, 1974, in national disgrace, he became the first President of the United States to resign. He boarded a plane with his wife and returned to his California home, ending his public career. A month later, in a controversial move, President Gerald Ford issued an unconditional pardon for any offenses Nixon might have committed while president. After a period of relative anonymity and when some criticism had softened, Nixon emerged in a role of elder statesman, visiting countries in Asia, as well as returning to the Soviet Union and China. He also consulted with the Bush and Clinton Administrations, and wrote his memoirs and other books on international affairs and politics. The Richard Nixon Library & Birthplace opened in the early 1990s in Yorba Linda, California. On January 20, 1994, in what would be his last public appearance, ceremonies honoring him on the 25th anniversary of his first inauguration were held.