Will 'build-down' stand up?

In the first real test of political support for arms control in the United States Senate this year, senators voted last October to table the freeze proposal; they also appeared ready to pass President Reagan's ''build-down'' idea before it was withdrawn from consideration.

No doubt both proposals, competing for political and public support, will face a full Senate showdown within the next few months.

President Reagan portrays his build-down as a significant step toward compromise with the Russians at the currently suspended strategic arms or START talks in Geneva.

One of the plan's Senate sponsors, Sen. William S. Cohen (R) of Maine, describes the President as having ''gone the extra mile'' in placing ''everything on the table'' with build-down. Indeed, the concept has already won bipartisan support in Congress and helped the controversial MX missile program garner a majority.

Yet critics describe build-down as a political diversionary tactic, an empty presidential promise intended to detract attention from the freeze. Sen. Mark O. Hatfield (R) of Oregon criticizes it as ''a general's dream'' which ''does not stop the arms race.'' The Russians meanwhile have flatly rejected it as a smokescreen for further US nuclear expansion.

On initial examination, the President's proposal appears to represent ''real reductions'' in nuclear arsenals. For every new nuclear warhead deployed, two would be dismantled. This ''two-for-one'' idea, proposed earlier this year by Sen. Sam Nunn (D) of Georgia, Senator Cohen, and others, would aim at reducing missile warheads by one-third to 5,000 on both US and Soviet sides.

The proposal includes another new negotiating card: a willingness to bargain numbers of bombers and cruise missiles, both conspicuously missing (as the Soviets quickly pointed out) in previous US START proposals.

Closer scrutiny of build-down, as reported in press accounts and congressional testimony, reveals glaring deficiencies which will not stand up to public attention. In fact, build-down, as now proposed, would heighten rather than dampen nuclear tension.

Build-down is not effective in limiting the qualitative arms race. Its purpose is to cap the quantitative race, yet in so doing it channels competition into more dangerous areas such as superaccuracy and concealment.

Build-down would allow, at a minimum, the following US systems: MX, Trident I and II, the new morile Midgetman strategic mini-missile, now in the design phase , and all later ballistic missiles, all cruise missiles, the B-1 and Stealth bombers, and the Trident submarine.

On the Soviet side it permits, indeed encourages, at least three new strategic systems - the SSX-24 ballistic missile, long-range cruise missiles, the Blackjack bomber, and additional Typhoon submarine production.

The cost of these systems alone will approach $500 billion. The freeze, by comparison, would stop these dangerous weapons from being deployed at all.

Build-down will undermine stable deterrence. The whole purpose of arms control negotiations is to deter nuclear war. Yet, by encouraging heavy competition in weapons modernization, the proposal does just the reverse: It bolsters the headlong rush by both nuclear powers toward first-strike and war-fighting weapons. Although overall numbers of warheads will drop, the percentage of those capable of counterforce and/or first-strike targeting will rise.

Under one build-down scheme presented in August by Rep. Albert Gore Jr. (D) of Tennessee, the ratio of US ''hard-target-capable ICBM warheads'' to Soviet silo targets would increase by a factor of three, thus making over half the Soviet force theoretically more vulnerable to a US first strike. At the same time, through continued testing and deployment of new missiles, the Soviets would be able to place US ICBMs at greater risk.

Build-down will undermine future arms control. In permitting cruise-missile deployments to progress, the proposal allows the deployment of a new long-range weapon which may not be adequately verifiable by satellite photography. It also includes two other dangerous loopholes: the lack of any constraints thus far on anti-submarine warfare (ASW) and antisatellite (ASAT) weaponry, thus failing to protect the critical sea-based deterrent and satellite-based intelligence. In fact, the US will be testing an ASAT prototype weapon this year.

Build-down seeks to restructure Soviet forces along American lines. Contrary to some views, the US continues to push Soviet military planners, historically and geographically more secure with land-based forces, toward sea- and air-based nuclear systems. The proposal does this by offering variable build-downs: ''two-for-one'' with land-based ICBMs, ''three-for-two'' with sea-based SLBMs, and ''one-for-one'' with mobile land-based missiles.

It thus favors submarines and mobile systems such as Midgetman, areas where the US has exhibited solid technical dominance over the Soviets. The Soviets rejected President Jimmy Carter's March 1977 proposal largely on these grounds; similar biases have no doubt led to build-down's recent rejection.

Build-down complicates rather than simplifies arms negotiations. By encouraging a qualitative arms race, the new plan makes our security more dependent on the satisfactory negotiation of increasingly complex treaties. If new destabilizing technologies were frozen, counting rules acceptable to both sides would be easier to establish and sustain.

In unveiling his new initiative, President Reagan has stated that ''the door to agreement is open.'' Unfortunately, the proposed build-down plan offers little in the way of serious, effective arms control, certainly less than does the bilateral nuclear freeze. Arms control through arms racing is not the solution.