Sen. Edmund S. Muskie stalked from his own Senate Budget Committee's April 3 session for good reason: his lavishly overpraised "budget process" had just been exposed as deeply partisan.

Muskie exploded when his hairshirt on the committee, Utah's freshman Republican Sen. Orrin Hatch, revealed an intimate aide's six-month-old memo to Muskie that exploded pretensions of nuetral objectivity in the new congressional budget process. It laid bare backstage maneuvers between Muskie's Budget Committee staff and the supposedly non-partisan Congressional Budget Office, intended to smother uncongenial ideas.

The most uncongenial idea around is that tax reduction can generate economic growth without inflation. The congressional budgetary machinery fiercely resists such anti-Keynesian hersey. Accordingly, Hatch last fall pressed for Budget Committee hearings to determine whether prestigious CBO Director Alice Rivlin was using outmoded economic forecasting models that obscured tax reduction's economic benefits.

Advice to Muskie came from a longtime trusted aide: John McAvoy, Budget Committee staff director. "Alice doesn't really want to have hearings," McAvoy told Muskie in an Oct. 4 memo, "and would like us to put Hatch off somehow. She says . . . that the critics of the models CBO uses for forecasting are an extreme right-wing claque who should not be given an audience, lest it legitimize their views and give Hatch a forum which ought to be denied him if we could."

A few days earlier, Hatch got the opposite impression from Rivlin, and wrote Muskie Sept. 27 saying that he thought she considered his proposed hearings "a valuable exercise." In fact, Rivlin wanted to stack any such hearings. "If we are to hold hearings," said McAvoy's memo, "Alice believes that they should involve noted economists telling the committee that Hatch's witnesses are wrong."

Such manipulation might shock admirers who call the budget a rare triumph of congressional reform in tempering Capitol Hill's spending excesses. In fact, the system has worked to depress military spending, preserve social welfare programs and stave off tax reduction; budgets, if balanced, would be balanced at a high level with no trimming of the welfare state.

This reality has been hidden in the Senate by the cozy arrangement between Muskie and the Budget Committee's senior Republican, Sen. Henry Bellmon of Oklahoma. But newcomer Hatch has been blowing the whistle in what McAvoy's memo called a "vindictive campaign against the process." By his critique of Muskie's budget operations published in Barron's operations published in Barron's magazine, said McAvoy, Hatch "puts himself beyond the pale."

During last year's Budget Committee consideration of defense spending, Hatch sniped at the committee staff's presentation -- especially a chart seeming to show NATO considerably stronger than the Warsaw Pact. During the congressional recess, Hatch visited Europe to reconfirm the well-known deplorable condition of the Western alliance.

So, Hatch complained at the Budget Committee session April 2 when the committee staff's summary of the previous year's hearings included that ludicrous NATO-Warsaw Pact chart, immediately following printed testimony of eminently non-dovish defense expert Edward Luttwak. It was "as though Dr. Luttwak was supportive of the charts," complained Hatch, who called it all "totally misleading."

Ed Muskie erupted. "Paranoid, conspiratorial," he shouted at Hatch. It was a bad day for the chairman. His committee raised defense spending limits against his wishes, leading Muskie to declare that now "I don't have to look for added [domestic] cuts."

The last straw came the next day when Hatch revealed the McAvoy memo, which he said came to him in an unmarked envelope presumably from a "mole" on Muskie's staff. Although McAvoy last October told Muskie, "I do not see how we can deny Hatch the hearing without giving him another cause of complaint about the committee and a chance to allege that we are covering up CBO's deficiencies," Muskie is made of sterner stuff. He denied it.

But Orrin Hatch is one senator who does not flinch under Muskie's assault. Immediately after their April 3 shouting match, he wrote Muskie renewing his request for hearings on economic models and gave it a presidential-year backspin: "Frankly, the Kemp-Roth tax reduction proposals that Gov. Reagan has now made part of his campaign program need to be examined by the kind of task force I have requested."

The last redoubt of Keynesianism is not anxious to validate supply-side tax theories of a Republician presidential contender. But if the budget process truly were a standard of professional objectivity as advertised, Dr. Rivlin would be out front asking whether tax reduction truly could point toward controlling inflation without crippling the economy.