For more than a decade, the U.S. attorney in Maryland has wielded exceptional influence over the life and politics of the state and even the nation, and the men who have held the office have been of one breed -- attorneys from the Baltimore legal establishment.

Although no one has been officially nominated to succeed outgoing Democrat Russell T. Baker Jr., the Reagan administration is expected to continue the line by selecting J. Frederick Motz, a Republican with the requisite school ties, pedigreed Baltimore law firm background and smattering of politics.

"That's Baltimore," said one former member of the federal prosecutor's office. "Every U.S. attorney in recent years has been connected in some way to legal establishment. If it comes to the choice between a politico and someone who is of the party in power and also part of the establishment, the establishment will win every time."

And so it was a surprise to very few when the FBI began conducting an exhaustive background check, the final hurdle before nomination, on Motz, and not on his closest competitor for the job, Baltimore County State's Attorney Sandra O'Connor.

Among those least shocked was O'Connor herself, who, along with pronouncing Motz an excellent lawyer, said this week, "I don't really think they'd break tradition . . . the tradition of naming someone from an establishment firm."

The U.S. attorney's job has not officially changed hands. Baker resigned the office, as is customary when the administration changes, and plans to take several months off before joining his old law firm, Piper & Marbury, Oct. 1. FBI checks on Motz probably won't be completed for several weeks, and he then would have to be officially nominated by Reagan and confirmed by the Senate. Meanwhile, Baker's chief assistant, Herbert Better, has taken over as acting chief.

Motz, a 1967 graduate of the University of Virginia Law School, is a partner in the silk-stocking law firm of Venable, Baetjer & Howard, where he is known as an exceptional trial lawyer with a low-keyed, "Jimmy Stewart quality" about him. His only connection to politics is as counsel to the state Republican Party.

Motz was clearly the legal community's favorite for the post, just as Baker, the son and namesake of a wealthy Baltimore real estate man, had been in 1977. Baker's background included a Harvard law degree, a stint with one of Baltimore's most venerable law firms and a job clerking for U.S. Supreme Court Chief Justice Warren Burger. Baker was once an assistant prosecutor in the office he was later to head, and for a time even shared a small office with another youthful assistant, J. Frederick Motz.

In those days in the early '70s, Baker learned firsthand of the office's immense power when he worked as the junior member of the team that investigated political corruption in Baltimore County -- a probe that wound up forcing Spiro T. Agnew from the vice presidency.

In prior years, the Maryland U.S. attorney had been responsible for convictions of congressmen, a U.S. senator and several local politicians. Later would come the political corruption conviction of Maryland Gov. Marvin Mandel, tried in the months just before Baker took office.

Baker knew only too well the act he had to follow, but said recently he wasn't expecting any new Mandels or Agnews when he took over. "Maryland politics had changed. There was an honest governor and an aggressive attorney general," he said as he relaxed at his Columbia home last week.

"What's important about that office is it's a Maryland institution with a heritage, a tradition of excellence that goes back a long way," the 39-year-old Baker said. "When I was sworn in, I said my goal was to continue that tradition, and to continue that was to shoot awfully high."

The clubby Baltimore legal community has given Baker's performance mixed reviews. Some have criticized the office for a "tough-guy" prosecutor attitude during Baker's early reign and for bringing major cases that didn't stand up in court.

"The office certainly had its ups and downs," said one prominent defense attorney, who pointed to one highly publicized down, the loss of a case against Baltimore State's attorney William Swisher, who was found innocent on charges that he sold that the powers of his office to a Baltimore political boss. "That case should never have been brought."

Although Baker concedes that he "made some mistakes," he does not believe the Swisher indictment was one of them. Indeed, he says he was "proud" of the case and the lawyers who handled it.

Other observes of the office say Baker simply had "bad luck"; that the big, political corruption cases his predecessors won just weren't there.

"If you happen to be U.S. attorney at a time when there is little or no corruption in Maryland government, then regardless of the quality of your staff or your leadership, there's not going to be the kind of World War III headlines, like 'Agnew Resigns,' as there were in the years before," one former assistant prosecutor said. "That's not something you blame the guy in office for. Perhaps it's something you should be grateful for."

Baker, in looking back on more than three years in office, said he is proudest of the staff he hired and his recruitment of many "bright, hardworking and dedicated" black and women lawyers -- without a quota program.

Asked what the high point of his term in office was, Baker said he would be hard pressed to pinpoint one case. "That office is such an institution in Maryland, do you say the highpoint was the savings and loan scandals of the late '50s . . . or in '64 when they took on [former speaker of the Maryland House] A. Gordon Boone . . . it was it when we chased Agnew and Mandel or when we went after Gsa?" he asked, referring to the scores of kickback convictions of General Services Administration officials won during his term in office.

It is this office, with its long and distingished record, that will be handed to Baker's successor, most likely the 38-year-old Motz, who was proposed for the job along with two other lawyers Sen. Charles Mc.C Mathias (R-Md.).Motz declined comment on his possible nomination, saying only, "I was told I was under consideration by the Justice Department."

That circumspect reaction is completely in keeping with the cautious Motz style described by friends and colleagues. But former U.S. attorney George Beall, a close friend, warned that no one should be fooled by Motz's "retiring, almost shy and softspoken" demeanor. "In the arena, he is all fighter," Beall said.

And Maryland Attorney General Stephen Sachs, who also once held the federal prosecutor's job, said Motz has "just the right mixture of care and zeal . . . a capacity for moral outrage" that the office requires.

But it is an office, too, that seems to make a mark on its occupants. "I think I have grown in the job. You'd have to be a cinder block not to," Baker said last week.

When he became U.S. attorney, Baker had a reputation for being imperious, even arrogant. Asked if the job mellowed him, he replied: "Yes, I have mellowed, but I don't want to talk about that." Pausing for a minute, he paced up and down the sunny patio.

"You know one thing that is not good is youth and brains and power," he said. "That is quite a brew, a heady brew. It can end up with arrogance. When I was a young assistant, I'm sure I was pretty hard to take."

But these days Baker seems to have changed. "My biggest surprise in the job was that there isn't always any clear right and wrong," he said. "When I was not the senior man, the answers always seemed much clearer. . . . Now I have had to watch myself when the facts and law aren't clear, but you just have to decide."