Overriding the objections of the US Defense Department, President Reagan has embraced a new approach for the reduction of strategic nuclear arms. His adoption of the ''build-down'' concept will improve his chances of getting appropriations votes for the big MX missile, members of Congress say. It will also be likely, they predict, to win some congressional supporters away from the nuclear freeze movement.
But it's far from certain how the Soviet Union will react. The Soviets' shooting down of a South Korean airliner five weeks ago has led to an increase in East-West tensions. The Soviets appear to be in a testy, suspicious mood at the moment.
''Digging in their heels'' is the way some specialists describe the Soviets' current posture. And the Kremlin is not known to adjust quickly to changes in United States arms control positions. So no immediate breakthrough is expected in either set of US-Soviet arms control negotiations now taking place in Geneva.
On Tuesday, Mr. Reagan announced his support for the build-down approach and for more flexibility in the strategic arms reduction talks (START) in Geneva. The chief US negotiator at the talks, Edward L. Rowny, was returning to Geneva on the same day.
The build-down concept would permit both superpowers to continue to modernize their nuclear arsenals. But for every new warhead that is added, they would have to destroy a specific number of older warheads. The idea would be to shrink the number of what are considered to be the most ''destabilizing'' nuclear weapons. The administration wants to encourage the development of small, single-warhead strategic missiles and move away from the heavy, blockbuster-type missiles on which the Soviets depend so much. The new Reagan proposal calls for a guaranteed build-down of about 5 percent a year.
Senators and congressmen who favor the build-down say that Reagan has been forceful in his support for the concept. One of its original sponsors, Sen. William S. Cohen (R) of Maine, says that Reagan called him to discuss the idea on the day he introduced it in a newspaper column 10 months ago. In the interim, Senator Cohen and Sen. Sam Nunn (D) of Georgia gathered support from 43 other senators and bargained intensively with executive branch specialists over what the trade-offs would be if the administration adopted build-down. The senators threatened to withhold support for MX missile appropriations, which come up for key votes within the next few weeks, unless the administration accepted the idea.
Senator Cohen and Senator Nunn, together with Senate Foreign Relations Committee chairman Charles H. Percy (R) of Illinois and three key House Democrats - known collectively as the ''Gang of Six'' on the issue - all seemed delighted with the outcome. To ensure that attention continues to be paid to the issue, Reagan appointed R. James Woolsey, a Democratic member of the bipartisan Scowcroft Commission on strategic forces, to the START negotiating team.
In a telephone interview, Cohen acknowledged that the Defense Department ''had not been terribly supportive'' of his concept. But he asserted that Reagan ''really has taken charge and demonstrated to a lot of doubting people that he is sincere about arms control.''
''The military is always reluctant to accept reductions,'' says Cohen. ''They're not paid to think diplomatically or politically.''
Other sources in Congress and the State Department say that in addition to Defense Department objections to build-down on various technical grounds, there was always opposition within the government to any negotiating idea that came from outside the executive branch. One official says there was considerable fear that Reagan's agreement to the build-down concept would set a precedent for growing congressional involvement in the details of US arms control proposals. A member of one of the Geneva negotiating teams complained that the ''Gang of Six'' kept making their plan more complex, advancing from build-down to ''son of build-down.''
Although few of his critics have given him credit for it, Reagan has over the past few months introduced a degree of flexibility into his original START proposals. His most significant concession has been to move away from an insistence on sub-limits for Soviet medium and heavy land-based missiles. This was what critics called the ''joker'' in the Reagan START position, because it would have required the Soviets to radically restructure their nuclear forces while not demanding the same of the American forces. Under the new Reagan package, the Soviets would be permitted to replace the requirement for sublimits with a more flexible limit on missile throw-weight, or destructive capability and potential.
Barry M. Blechman, a widely known defense consultant who until recently has been highly critical of Reagan's START proposals, says he sees the new Reagan package, and the build-down concept in particular, as a significant step in the right direction.
''The beauty of the build-down is that it forces you to consider arms control and weapons programs together,'' says Mr. Blechman, who served as an assistant director of the US Arms Control and Disarmament Agency under President Carter. ''Reagan kept talking about deep cuts in nuclear weapons, but he had a weapons program that simply led to a big buildup.''