When Secretary of Defense Harold Brown fought a losing battle against congressional approval of a new chemical warfare plant, he reversed his own instructions to Pentagon deputies and showed the continuing influence of the arms-control lobby on Carter administration policy despite heightened tensions between Washington and Moscow.

The Senate followed the House last week and approved the new plant over brown's objections. That served notice on President Carter that Congress will no longer accept Brown's submission to the arms controllers. The rebuff for Brown, coming in the midst of charges that he is the most politicized secretary of defense ever, reflected congressional rejection of the arms-control-theology that has dominated policy since Carter took office.

The detailed story of Brown's obersance to that theology, despite overwhelming evidence of a Soviet chemical warfare buildup, is yet another piece of evidence that the administration did not truly change following the Soviet invasion of Afghanistan. The rhetoric has turned hard, but the policy remains soft.

The story of the small ($3 million) authorization for the first new chemical warfare plant in decades started last Oct. 12. Undersecretary of Defense William J. Perry sent a confidential memorandum to Brown saying that "in response to your request, a game plan has been developed" that would "assure" inclusion of the plant in the new defense budget.

About the same time, Brown himself wrote national security advisor Zbigniew Brzezinski that he had "directed" the Army to request money to start building the plant. But just before the Senate began debate earlier this month, Brown informed Sen. John Stennis, chairman of the Senate Armed Services Committee, that the project was "premature."

What changed Brown's mind was the administration's arms-control theology. When Perry advised Brown last Oct. 12 that plans for the chemical plant were on track, he also warned that "a key part of the game plan involves concurrence of the other concerned agencies." That meant the Arms Control and Disarmament Agency backed by detentists in the State Department. They refused to concur.

The arms controllers were still clinging to a confidential letter written by then-Secretary of State Cyrus Vance Oct. 23, 1977. Vance stated the administration's arms-control theology in terms now made laughable by Soviet military aggression and Soviet treaty-breaking.

Arguing against any effort by the United States to catch up with the Soviet chemical warfare lead, Vance said the United States would gain a "significant psychological advantage" over Moscow with restraint showing "our sincere intent to limit offensive weapons."

Embracing Vance's obsolete theology, the arms controllers persuaded Carter this year. They argued that repeated Soviet violations and new evidence of Soviet foot-dragging over arms-control negotiations should not stand in the way of Vance's admonition for "restraint."

The Senate rebelled last week. Led by Democratic Sen. Henry Jackson and Sam Nunn and Republican Sen. John Warner, a bipartisan defense bloc built a masterful case against the Carter policy. Voting with the bloc was one of Carter's most faithful loyalists, Chairman Stennis. In a rare departure from that loyality, Stennis told the Senate that "in view of the undoubted change in the atomosphere" between the United States and the Soviets, the United States could not risk further delay.

The congressional order for this belated effort to start narrowing the immense Soviet lead in the ghastly arts of chemical warfare would seemingly be enought to jolt Brown out of subservience to the arms controllers. Leaders of the defense bloc doubt it; if recent Soviet actions could not free Brown, nothing ever will.

In the face of U.S. "restraint" on the nuclear front, the latest Soviet underground test Sept. 14 is acknowledged even by arms-control officials to have greatly execeeded the 1974 Test [WORD ILLEGIBLE] Treaty limit.

But that is dwarfed by evidence that Soviet forces are using chemical warfare to subjugate Afghanistan and that Moscow's ally, Vietnam, is doing same in Laos and Cambodia. After four years of U.S. restraint in trying to negotiate a chemical warfare ban with Moscow in Geneva, the first sentence of a treaty has yet to be agreed upon.

During those four years, Carter and Brown have kept the lid on U.S. production. (Not one chemical warfare shell has been produced for 10 years, none fired for 11.) The Kremlin has used those four years to arm up to one-third of all its artillery and rockets in Central Europe with chemical warfare shells.

Testifying last spring, Gen. Bernard Rogers, the NATO supreme commander, warned he has "no capable retaliatory [WORD ILLEGIBLE] . . . to deter [Soviet] use" of its arsenal. The wonder is that at this late hour, Congress has to force the secretary of defense to start correcting that deficiency.