SINCE World War II, no living American governmental leader has been such a consistent fixture on the United States scene as Richard M. Nixon. Rising from middle-class roots in Yorba Linda and Whittier, Calif., Mr. Nixon served his nation as a congressman and senator, vice president, and finally, president. Forced to resign following his coverup in the Watergate scandal, Nixon went not into quiet retirement, but assumed the role of "elder-statesman," sought out by US and world leaders alike.Stephen E. Ambrose, a professor of history at the University of New Orleans, and an expert on the Eisenhower presidency, now follows his two excellent earlier biographies of Nixon - from childhood years to the White House - with a concluding volume, although in the case of the 37th president, the word "conclusion" seems to have little relevance. Nixon may have left office under a cloud; but there he is, many a night, on the evening news. Volume III explores Nixon during his period of greatest crisis, from 1973 to 1990. The book never manages to pin down the one nagging question from the early 1970s: Why did the president's men break into the offices of the Democratic National Committee at the Watergate? And Ambrose moves perhaps too quickly through the issue of Nixon's personal involvement in the break-in, or other break-ins. But these are minor quibbles. Volume III is an admirable study of the triumph of the American constitutional system over the worst possible excesses of duplicity and ambition at the highest levels of power. The three volumes are judicious, informative, and deeply moving. Ambrose's work will surely be the definitive biography of Richard Nixon for many years. As pictured by Ambrose, Nixon is a deeply troubled, self-serving, and often sinister individual. Indeed, Volume III opens with Nixon, shortly after his triumphal reelection in 1972, plotting the downfall of his enemies. Nixon is ever the manipulator. Spiro Agnew, his vice president, is forced to leave office following disclosures of graft. When Nixon finally taps a vice president, his selection is Gerald R. Ford because of Ford's popularity with his fellow congressmen, which, Nixon surmises, might help him survive Watergate. And when Nixon finally leaves office, he flees at the last possible moment. "No Administration in American history was more ruthless, more partisan, more personal, or more reckless in its disregard not only for the law but for the decent opinion of mankind as that of Richard Nixon," Ambrose says. Ambrose minces no words about Nixon's flaws: Still, he obviously has a deep affection for his subject. Nixon was blessed with a fine, supportive family. He was and is a man of refinement and intellect. Politically, Nixon stood somewhat to the left of Eisenhower, although to the right of many Democrats (and some Republicans) of his era; one tends to forget Nixon's activism - a form of social-welfare Republicanism that ended with his resignation in 1974. Nixon called for a minimum payment to families with children, federal revenue sharing, and the abolition of the draft. He set up quasi-independent federal agencies to run the post office and passenger railroads. He created the Environmental Protection Agency. He set up a Pay Board and Price Commission to control inflation. And in foreign affairs, US involvement in Vietnam finally came to an end; he sought detente with the Soviets; and Washington established diplomatic ties with China. Nixon was undone by his own excesses. He could have been a great president. "As it is, he doesn't even rate as a good one," Ambrose says. When he resigned, argues Ambrose, the Republican Party, and the US, moved to the right - setting in motion the political conservatism of the 1980s. "When Nixon resigned," Ambrose says, "we lost more than we gained."