The Republican Senate staffer who was fired for supposedly offending ex-senator Mike Mansfield, U.S. ambassador to Japan, in fact made the mistake of disrupting cozy arrangements within the congressional bureaucracy.

Just why Dr. Michael Pillsbury was sacked from the minority staff of the Senate Budget Committee is shrouded in contradictions, denials and even neo-McCarthyism. But Pillsbury might still be on the job had he not run afoul of hand-holding between staffers of the two parties.

While minority staffers in Congress have multiplied dramatically, they are all too often collaborators with their majority brethren rather than adversaries. Nowhere is this more true than on the Budget Committee, where the chairman, Sen. Edmund Muskie of Maine, and the senior Republican, Sen. Henry Bellmon of Oklahoma, work as one.

To defense-oriented senators of both parties, the Muskie-Bellmon entente results in the restrictions on defense spending in the name of economy without benefit of Republican opposition. It surprised nobody that David Shilling, the minority staff member of the Budget Committee supposedly representing Republican defense views, this year received a Carter administration political appointment in the Pentagon.

His successor was Pillsbury, a Chinese linguist and Far Eastern specialist who most recently was a research fellow at Harvard. After Shilling endorsed him, Pillsbury was picked by Bellmon's minority staff director, ex-Kansas City stockbroker and investment banker RObert Body. A partisan Republican (unlike his predecessor), Pillsbury immediately raised hackles by cooperating with defense-oriented Republicans, led by Sen. John Tower of Texas, on the Armed Services Committee.

How long Pillsbury would have lasted with such behavior is debatable, but his fate was sealed when he went on a previously planned Far Eastern trip a few weeks after starting his new job. His conversations with Japanese officials, with a U.S. embassy escort sitting in, aroused the ire of Mansfied, the longtime Senate majority leader.

Mansfield wrote a "Dear Ed" letter to old friend Muskie, which was cabled to the State Department with "confidential" security classifications. The ambassador criticized Pillsbury's "free-wheeling approach" in Japan, accused him of "a strong pro-PRC [Communist China], anti-Soviet bias" and bridled at alleged statements by Pillsbury that Mansfield was out of touch with Congress. Nowhere in the three-page, single-spaced letter did he ask that Pillsbury be sacked.

Nevertheless, Pillsbury was ordered home immediately from South Korea and was fired by Boyd. In probing the reasons, the story becomes murky, but filled with overtones of covert bipartisan collaboration.

Mansfield's cable to Muskie was hand-delivered by the State Department to John McAvoy. Buget Committee chief of staff, while Muskie was up in Maine, McAvoy took it to Boyd, who then called Pillsbury home. According to Pillsbury, when he arrived in washington Boyd told him that McAvoy insisted on Pillsbury's resignation before he showed the cable to Muskie because the senator might act in "an unrehearsed manner" - a reference to Muskie's fabled temper.

Boyd and McAvoy vigorously deny that, and State Department officials insist Mansfield did not cause Pillsbury's fall. Furthermore, Boyd told us there was nothing in his brief tenure on the committee staff to cause his dismissal.

What, then, caused the fuss? Boyd evoked Kafkaesque or McCarthyite overtones by saying he received (after Pillsbury left for the Orient) unspecified derogatory information about Pillsbury's past that revealed "a pattern of behavior that was unacceptable" and "poor interpersonal relations." Just what was wrong he would not say but suggested we ask about Pillsbury's record at the Rand Corporation.

From checks with colleagues both in and out of Rand, this picture emerges: Pillsbury is brilliant, innovation, but sometimes abrasive. His record was spotless before arriving at Rand's think tank in Santa Monica, Calif., in 1973. While there, according to one ex-associate, Pillsbury performed "some of the most interesting work on China" but proved "inadvertently threatening to jealous colleagues." In short, "his profile was just a little high."

That same grievous shortcoming in the age of the organization man plagued Pillsbury on the Budget Committee. Some Republican senators on the committee, who agree with the late Robert Taft's admonition that it is the business of the opposition to oppose, believe that Democrats dictated the dismissal of a GOP aide and are outraged.

Senatorial courtesy to Bellmon, however, prevents those senators from letting us use their names, just as it prevents their openly attacking the system. For that reason, the system that chewed up an innovative though prickly personality and muffles the Republican voice of opposition is unlikely to change.