White House lowers its voice on alleged arms violations

As the critical issues of nuclear weapons, superpower relations, and arms control converge, President Reagan once again finds himself squeezed between his political instincts and the complexities of governing.

Conservative Western senators, among the most solid of presidential backers, are urging Mr. Reagan to advertise alleged Soviet violations of some provisions of SALT agreements (the Strategic Arms Limitation Treaties). Sen. James A. McClure (R) of Idaho released new data Monday in what has become a litany of such charges.

Yet Mr. Reagan has softened his own rhetoric on the subject and is proceeding cautiously. There are two reasons for this.

Even those critical of the administration's policies on weapons and arms control acknowledge that the Soviets have ''pushed to the allowed limits.'' Yet, as senior defense intelligence officials and those who have been at the bargaining table in Geneva note, the ambiguities in existing arms-control agreements make it difficult to prove that MoscowufdaveNew US arms stand urgedWS 8

is breaking the restrictions on testing and deploying new strategic weapons.

The administration is also constrained from challenging the Soviet Union on the issue at this time because of concern about its own plan to modernize United States strategic might. Officials agree, for example, that deploying both the MX missile in existing silos and building a new, single-warhead mobile missile (as recommended recently by the special commission on strategic forces) would violate SALT II. This agreement has not been ratified, but the US has agreed to honor it if the Russians do.

There are other political considerations as well. The administration last week finally got Kenneth Adelman, its nominee to head the Arms Control and Disarmament Agency, confirmed by the Senate. With the nuclear-freeze resolution and MX funding still pending in Congress, the administration must indicate its seriousness about arms control. Bellicosity on a debatable US-Soviet issue at this time would not help the administration's cause.

The main charge against Moscow is that it has been testing two new ICBMs, while allowed only one under SALT. US intelligence agencies detected test firings in October and in February, and some say that the missiles were different. The Soviets deny this, and there are complicating factors.

In developing conventional and nuclear weapons, the Soviet Union has a history of proceeding in incremental stages rather than developing a new generation before deployment. Some officials and analysts interpret SALT II to allow a certain amount of change in new missiles during the first 12 test launches. Senior military intelligence sources acknowledge this in describing recent Soviet activity.

''Is it the test of a new ICBM or a modified ICBM?'' former chief SALT II negotiator Paul Warnke asked recently. ''There is no hard evidence that they have violated SALT II.''

In its report on the Soviet Union and arms control, the Carnegie Endowment for International Peace this week concluded that Moscow has not violated SALTs I or II. But it noted that the Soviets ''press at the ambiguities in limitations, '' and said that ''preliminary reports of Soviet tests of two new ICBMs are a current cause for concern.''

Part of the problem of verification is the degree to which missile test data are encoded during transmission to control facilities. Much of such data is supposed to be transmitted without secret coding so that the other side can verify test-missile range and payload. Senator McClure charges that the Soviet Union has been encoding too much data in an attempt to hide test results from the US.

Drawing on government intelligence reports, McClure also says the Soviets have violated existing treaties by developing a rapid reload and refire capability for its heavy ICBMs; stockpiling extra missiles to circumvent limits on launchers; deploying a mobile ICBM; conducting underground test explosions beyond the limit of 150 kilotons; deploying long-range cruise missiles on Bear and Backfire bombers; building more Backfires than allowed; and testing a surface-to-air missile designed to destroy incoming ICBMs.

At his press conference last Friday, President Reagan acknowledged the ambiguities in existing treaties, saying it is ''difficult to establish . . . hard and fast evidence that a treaty has been violated.'' National security adviser William P. Clark is heading an interagency study of possible violations.

As a presidential candidate, Reagan in essence made the same charges as McClure and said SALT II was therefore ''fatally flawed.'' Many charges made by McClure and others have been detailed or hinted at in such documents as the Defense Department's recent ''Soviet Military Power'' report. For now Reagan is lowering his voice on the subject.