Through the four years and four months of the Nixon administration, Harry Robbins (Bob) Haldeman has been the least visible and most powerful of that small band of White House assistants who could claim to be close to President Nixon.

At the White House he was the chief of staff, and from that position he wielded enormous power that flowed directly from his absolute control over both the people and the paper that reached the President’s inner office.

He was aided in the exercise of that power by a personal relationship with Mr. Nixon that had been fostered over years of working together, stretching back to the mid-1950s and the Nixon campaign for re-election as vice president. Said to be Mr. Nixon’s closest and most trusted aide, Haldeman is intensely loyal to the President and in Washington gathered around himself a group of tight-lipped young men who shared his devotion to the President and, in turn, were absolutely loyal to Haldeman.

Among them were former presidential appointments secretary Dwight L. Chapin, deputy Nixon campaign director Jeb Stuart Magruder and former presidential assistant Gordon Strachan -- names that kept cropping up in the investigations of the Watergate bugging incident and related allegations of political espionage and sabotage during the Nixon re-election campaign in 1972. Chapin, Magruder and Strachan have left the administration.

Slowly, the links between these loyal Haldeman aides and the ever-growing Watergate scandal became public. As late as April 4, Sen. Sam J. Ervin (D-N.C.), the chairman of the Senate select committee investigating the Watergate incident, issued a formal statement saying that “as of this time “ there was “no evidence of any nature” to link Haldeman personally with any illegal activities during the 1972 campaign.

Nevertheless, sources familiar with both the Senate committee investigation and a separate federal grand jury probe into the bugging said investigators considered the role of Haldeman as the key to understanding the undercover activities of the 1972 Nixon campaign.

As far back as last October, a Justice Department source, referring to the espionage-sabotage campaign, said, “This is a Haldeman operation.”

It was a curious position for Haldeman, who in almost 20 years of association with Mr. Nixon has been a largely invisible technician, a manager of people and paper flow who rarely voiced his own political convictions and who cherished, above all else, efficiency and his own personal anonymity.

Haldeman had stayed in the background through the first Nixon administration and the beginning months of the second as his power in the White House grew steadily. A Christian Scientist who neither smokes nor drinks, he would rarely show up in photographs of White House social functions or informal get-togethers of Nixon aides. He accumulated and held his power through hard, grinding work, arriving early at the White House and staying late. “Work consumes most of my father’s time,” Haldeman’s son, Hank, 19, said in an interview last year.

And, according to most accounts, he exercised his power ruthlessly. Best known for his trademark, the closely cropped, 1950s style crewcut, Haldeman in the White House was portrayed as an unsmiling, curt taskmaster who guarded access to the President so closely he discouraged Republican senators and Cabinet secretaries from even trying to see Mr. Nixon. In one of the most often repeated Haldeman remarks, former White House speechwriter Richard Whalen, in his book “Catch the Falling Flag,” quotes Haldeman as saying: “Every President needs an S.O.B. -- and I’m Nixon’s.”

They first met -- the President and self-described “President’s S.O.B.” -- in 1951 when Haldeman, on his first trip to Washington, visited Richard Nixon, the senator from California. Haldeman was fascinated by the Alger Hiss case and Sen. Nixon’s involvement in it.

Besides an interest in anticommunism, Haldeman and Nixon shared a common area of birth and upbringing -- Southern California. But other than that accident of geography, there was little in common in the backgrounds of Nixon, the son of the poor Whittier, Calif., grocer, and Haldeman, oldest son of an upper-middle-class Los Angeles businessman.

Haldeman, 47, was born on Oct. 27, 1926, in Los Angeles. His grandfather had moved to California from Indiana shortly after the turn of the century and founded a pipe and building supply company that Haldeman’s father later headed. His grandfather also helped found the Better American Foundation, an early anticommunist organization.

Young Haldeman attended private schools -- Hawthorne Grammar School and Harvard Episcopal School in Los Angeles -- then went on to the University of Southern California and, after a stint in the Navy, to UCLA, where he earned a degree in business administration. His roommate at UCLA was John D. Ehrlichman, head of the President’s Domestic Council.

In 1949, Haldeman married Joanne Horton, whom he had met at UCLA and whom a friend describes as a “quiet, reserved, intense, lovely woman.” They have two daughters and two sons, ranging in age from 12 to 21.

Haldeman first worked for Mr. Nixon as an advance man in the 1956 campaign. In the unsuccessful 1960 Nixon presidential campaign, Haldeman was elevated to chief advance man.

In between Haldeman returned to the J. Walter Thompson advertising agency in Los Angeles, where he was a vice president of the firm with accounts such as Walt Disney, 7-Up and Black Flag Insect Spray.

In 1962, Mr. Nixon recruited Haldeman again, naming him manager of his campaign for governor of California. This was the most disastrous Nixon campaign of all, ending in defeat and a tirade against the press (”You won’t have Nixon to kick around any more”) that seemed at the time to have stripped the former vice president of any hope of every again holding public office.

That 1962 campaign was all but forgotten during Mr. Nixon’s later successes until last October, when it became widely known that the 1962 Nixon gubernatorial campaign committee had organized and financed an effort to sabotage his opponent’s campaign among registered Democrats.

The evidence was in an official 1964 judgment of San Francisco County Superior Court that held that the effort was authorized by both Mr. Nixon and Haldeman.

The California judgment gave further credence to Haldeman’s reputation as the complete political pragmatist who gets the job done. Since he rarely speaks in public, little is known of Haldeman’s own political convictions, which he has described as “right of center.”

Most of what is accomplished is done by a few achievers rather than the general population,” Haldeman said in a 1970 interview. “I want to make sure we don’t enforce a common level of mediocrity by putting through programs that tend to lower everybody to a standard which can be attained by the majority.”

In the 1968 Nixon presidential campaign, Haldeman was given the title of Mr. Nixon’s personal chief of staff. After the election, he headed planning of the White House staff, putting together the people over whom he would exercise the most direct, day-to-day control.

Haldeman had no official title in the 1972 Nixon campaign, but was considered the architect of the President’s campaign. He continued to put in long hours at the White House and to accumulate thousands of feet of motion picture film he has taken over the years of the President and the Nixon family.

On Oct. 25, The Washington Post reported that Haldeman was one of five high-ranking presidential assistants authorized to make payments from a secret Nixon campaign cash fund. The fund, which at times contained as much as $700,000 financed a spying and sabotage campaign against Democratic presidential candidates, according to federal investigators.

The White House denied at the time that a secret fund existed and said that “at no time did Bob Haldeman have authority to disburse or direct the disbursement of funds contributed for the President’s re-election.”

Haldeman’s role, if any, in the actual Watergate bugging and the campaign of political espionage and sabotage is not known, even today. But it is known that many of the men alleged to have been involved in the espionage and sabotage -- Chapin, Magruder and Strachan, for example -- had worked for Haldeman and owed their positions and loyalty to him and the President. Haldeman, in effect, ran the President’s re-election committee through these trusted aides.

On March 28, Haldeman told an off-the-record meeting of several Republican congressmen that he personally ordered “surveillance” of Democratic presidential candidates, including the taping of their speeches and public statements. At the meeting, he gave the impression that these supposedly legal activities somehow “got out of hand,” according to one congressman.

Haldeman, however, was deeply implicated in allegations of attempts to cover up White House involvement in the Watergate affair. At least two high level White House officials have concluded that the cover-up was supervised by Haldeman and his college roommate, domestic policy adviser Ehrlichman.

Over the weekend, reliable sources told The Washington Post that White House counsel John W. Dean III intends to swear under oath that he gave regular reports on the cover-up operation to Haldeman and Ehrlichman at their direction.

The Watergate grand jury also is attempting to determine whether Haldeman ordered the payment of “hush money” to the seven convicted Watergate conspirators in return for their silence, according to government sources. Payments to the conspirators allegedly came from a $350,000 campaign cash fund kept in the White House under Haldeman’s control.