In the fall of 1975, when suspended Gov. Marvin Mandel was trying to find some reprieve from his imminent indictment for political corruption, he sent the Justice Department an extraordinary 19-page letter condemning, among others, the Beall family of Western Maryland.

Mandel theorized, without providing any evidence, that for five years J. Glenn Beall Jr., then a U.S. senator, and his brother George Beall, a former U.S. attorney, had conspired with a handful of Democrats to destroy Mandel by abusing the powers of the U.S. attorney's office in Baltimore and manipulating the press.

The ploy was a complete failure. Mandel was indicted and then convicted last year. Now, Mandel's first elected successor may well be his old target, J. Glenn Beall Jr., the overwhelming favorite in the Republican gubernatorial primary.

Beall has come back to elected politics this year, after his 1976 defeat by Democratic Sen. Paul S. Sarbanes, as the hand-picked gubernatorial candidate of the state's GOP leaders. His brother George, the successful prosecutor of the former Vice President Spiro T. Agnew and a pioneer of political corruption investigations in Maryland, is his campaign chairman.

Already, Glenn Beall is campaigning around Maryland as if he has won the primary election - which polls say he will do, handily. He is looking beyond the primary, fully expecting to be the party's standard bearer against the 8-year-old Democratic administration that was led by Mandel.

In addition, while he shakes hands at county fairs, or visits local Republican clubs, Beall. Also, is demonstrating his ability to survive, and survive handsomely, his own painful 1976 defeat and others, attempts to discredit his family, who for generations have been leading citizens of western Maryland's Allegany County.

If there is any secret to his resiliency, it is Beall's geniality and the middle-of-the road Republican philosophy he has held to throughout his six years in the Maryland state legislature, his two years as a congressman and his six years in the U.S. Senate.

In this year when Democrats are espousing fiscal conservatism, Beall is the only gubernatorial candidate of either party who consistently mentions keeping "programs that are sensitive to the needs of the people" - housing, welfare and jobs programs - at the same time as he promises to lower taxes.

Beall also is the only major candidate in Maryland's history to choose a black running mate, Aris T. Allen of Annapolis. With Allen's help and the help of his own middle-of-the-road philosophy Beall hopes to rely on friends he has made in both parties to do battle against the Democrats.

This international appeal to a broad spectrum of voters comes as much from Beall's family background as the obvious dilemma any Republican faces in Maryland where the GOP is outnumbered by a 3-to-1 voter registration. It also is a lesson learned from his only defeat, the 1976 Senatorial election where Sarbanes was the clear favorite months before he won over the incumbent Beall by 230,000 voters.

In his election night concession speech, Beall made the traditional vow never to run for elected office again, a promise that, at the time, seemed sincere. The campaign had been a hard one for Beall, running as a Republican when the Watergate scandal still was fresh in people's minds and facing charges about his own ties to Nixon.

In 1976, Beall accepted $250,000 in campaign contribution from the Townhouse operation, a secret, White House-controlled fund that was funnelling money to certain Republican candidates for Congress. Both President Nixon and Vice President Agnew campaigned in the state for Beall and in November he won out over incumbent Democrat Joseph Tydings.

But the things that had been such assets in his 1970 campaign became the evil demons of his campaign efforts six years later. In that race, Beall's opponents described him as a man tainted by these associations, even after he admitted during the campaign that he made a "mistake" by accepting the Townhouse money.

He repeatedly contended then that he was a victim of post-Watergate morality, imposed six years after the fact. But he was soundly defeated in that race, and when it was finished he said he would never run again for anything.

As late as last April, he had not changed his mind. But one after another, the other potential candidates were backing off, and Republican leaders were turning to him. He entered the race in June, after Anne Arundel County Executive Robert Pascal and Beall's brother George bowed out.

"I missed government but missing office is one thing and running is another," he continued. "I asked myself if I should run any more, and I answered only if I had a chance at being elected. I'm not running to be defeated. I learned how painfull that can be.

Friends said privately that Beall, 51, was worried that his challengers would revive the old charges about Townhouse and set him off on a rerun of 1976. The two long-shot Republicans in the primary, Carlton Beall (who is no relation) and Ross Z. Pierpont, have done just that, but few people are listening.

The two Republican opponents have lifted a series of charges directly from the 1975 Mandel letter to the Justice Department. Not only Beall's campaign contributions from the Townhouse operation but his family's profitable move in giving up stock in a racetrack 17 years ago were raised by Mandel and have been raised again by the challenges in this campaign.

In this race, Beall has the assistance of his brother, George, to answer the charges. "On the Democratic side," George Beall says, "there has been a decade of dishonor in the state. But there's been enough dishonor to go around on both sides. The exposure of Maryland's political system - that it was rotten to the core - was begun by a Republican U.S. Attorney named George Beall."

This united front of the brothers was absent from the 1976 campaign in which George Beall played a fairly monor role. This time around, it seemed to make sense to the Bealls to go it together. "We don't try to influence each other," Glenn Beall explained. "I knew George wanted me to run but he never told me to. We just talk. We're always there to talk to each other."

They were both sons of the man most voters called "Senator Glenn," a former state senator, road commissioner, congressman and U.S. senator. Both brothers agreed that the Senator's namesake, J. Glenn Beall, clearly was the child selected to carry on the Beall tradition, both in politics and in business.

All three of the Beall sons were sent to preparatory schools. Glenn went to Phillips Exeter and graduated from Yale after a stint in the U.S. Navy. He then returned to Frostburg to take up his father's insurance business - which he still runs - and to make his way in politics. His first victory was in 1962 when he won a seat in the state House of Delegates.

As one of his opponents puts it, "the Republican Party has been good to the Bealls. Your father was United States Senator. You were United States Senator and George was United States Attorney." At the same time, the Bealls believe they have also been good to the Republican Party.

In 1970, then-Congressman Glenn Beall, captured a Senate seat held by another scion of a Maryland political family, liberal Democrat Joseph D. Tydings.

Because of that victory, Republicans held both of Maryland's Senate seats for six years. Sen. Charles McC. Mathias, the other Republican, had won his position two years earlier, vacating the Congressional seat Beall had captured in 1968.

Beall was a different type of Senator than the liberal Mathias. He concentrated on local issues and constituent work, answering every letter and earning a moderate rating from both the liberal Americans for Democratic Action and the conservative Americans for Constitutional Action. The strategy broke and Republicans throughout the state suffered from the party's disagrace.

In Maryland, the Republicans' predicament was underlined by the resignation of then-Vice President Spiro T. Agnew, Maryland's former governor, who pleaded no contest to a single count of tax evasion following his prosecution by George Beall, who was then U.S. attorney.

Friends of both brothers believe that George, now a private attorney in Baltimore, is working diligently for his brother because of a nagging concern that he somehow contributed to Glenn Beall's defeat in 1976. "It had to cross George's mind that his hour of glory [in the Agnew case] had a bomerang effect, warranted or not, when Glenn lost to Sarbanes," a friend suggested.

Now George Beall is using his own prosecutorial record to help his brother's candidacy. "Glenn should get some deflected credit for our attempt to clean up the system," he says. "People should know that Glenn's brother is a guy who tried to do something about corruption."

The otherwise private George Beall also has helped put together Glenn Beall's campaign staff. It was set into place only six weeks ago, almost one year after the Democrats began their campaigning. Heading the organization is John Shales, a member of the White House staff of former President Gerald R. Ford. Another alumnus of the Ford White House, Doug Bennett, is the cochairman of the Beall campaign.

Party leaders who convinced Beall to run this summer have given him their tacit or public support. They now are talking about a "new Glenn Beall", a man rejuvenated after his two years in private life. It is the sort of flattery one expects to hear from party officials who believe Beall is their only chance at a victory in the first post-Mandel gubernatorial election.

Beall agrees that his two years out of office helped his confidence, "gave me time to reflect." It also allowed him to improve his insurance business and shore up his personal finances. His estimated net worth now is more than $500,000 according to his financial statements.

Now, after a leisurely life filled with Georgetown dinners and an occasional trip to the Kennedy Center, Beall says he has "new energy." It is an image his supporters like to encourage. Gordon Hawk, an aide to Mathias, summed up the "new Beall" after attending the recent fund-raiser. "What I saw the other evening, was a Glenn Beall with more zip than I had ever seen; he was rolling."