Boston — One of the nation's best-known Republicans, Elliot L. Richardson, is back home in Massachusetts on the trail of a new job. The Harvard-educated former Bay State lieutenant governor, who may have held more high-ranking federal posts than anyone in the nation's history, now hopes to become a senator.
Citing ''the preservation of peace'' as ''the most urgent issue of our time, '' Mr. Richardson March 19 formally launched his candidacy for the senatorial seat wrested from the GOP six years ago by Democrat Paul E. Tsongas.
With the latter having decided for health reasons not to seek reelection, Massachusetts Republican activists are optimistic their party can win back the chair.
Despite his long government-service background in both elective and appointive posts, Richardson is by no means assured of reaching the November ballot. He must first win the September Republican primary in which he will be up against Raymond Shamie, the self-made millionaire industrialist who in 1982 was the party's challenger to US Sen. Edward M. Kennedy (D) of Massachusetts.
There is little agreement within Republican ranks here as to which of the two would-be senatorial nominees would be the stronger contender. Backers of Mr. Shamie, a conservative, say he would stand the best chance, since a liberal Democrat is expected to be the opposition party's nominee.
Richardson is a political moderate. And although he is supportive of President Reagan and the direction in which he has been taking the country, Richardson makes no secret that there are areas in which he does not agree with the GOP chief executive.
Richardson holds he is better qualified for the Senate seat. His experience in high federal positions under five presidents includes: undersecretary of state; secretary of health, education and welfare; secretary of defense; attorney general; ambassador to Great Britain; and secretary of commerce.
''I am proud of what I have done, and I am proud of what my conscience would not let me do,'' he said in his candidacy declaration. The latter statement was an obvious reference to his role in the October 1973 so-called ''Saturday night massacre,'' in which he resigned as US attorney general rather than fire special Watergate prosecutor Archibald Cox when ordered to do so by then-President Richard Nixon.
Richardson's move was widely heralded at the time, and his boosters view it as a particular asset in his voter appeal, especially among Independents and Democrats.
Shamie backers and other critics of Richardson consider him too liberal and not enough of a party loyalist. They also question his interest in Massachusetts , since for the past 15 years he has been out of state in various federal assignments or in private law practice and other ventures.
''Massachusetts has always been, and will always be, my home,'' he responds.
In pledging to be ''a senator for Massachusetts'' rather than just from the Bay State, the former Cabinet official emphasizes that his ''experience has taught me how to get things done in Washington.''
Despite his prominent roles in the federal government, Mr. Richardson has not been active in statewide political affairs since 1966 when he was elected state attorney general in a bitter and hard-fought campaign against Democrat Francis X. Bellotti, who later won the office.
''I will know how to tackle and reduce the huge federal deficit, to make selective cuts in the defense budget, and go about reducing the threat of nuclear war,'' Richardson said.
In the latter direction, he suggests the United States should ''intensify efforts first to freeze, and then reduce the number of nuclear warheads.'' The superpowers' goal, he says, should be to ''pursue a more stable balance among the weapons in each side's nuclear arsenal.''
The Richardson candidacy, which some observers suggest could help President Reagan to carry Massachusetts in the November election, was encouraged by leaders of the Reagan-Bush reelection committee.