Next six months crucial for SALT II. Despite battle within Reagan administration over pact, pressures mount to keep it

President Reagan continues to strike rhetorically at the SALT II treaty. For the moment the pact remains alive, but the next six months or so will be crucial in determining the ultimate future of the treaty. Arms control advocates say that hard-liners within the Reagan administration appear to be winning in the long internal battle over whether to keep the agreement, which the Senate never ratified. [Moscow's reaction, Page 7.]

But at the same time, there are growing pressures on the administration to stick with SALT II:

The Western allies are strong supporters of SALT II, which remains the sole framework for maintaining the nuclear arms balance and reducing the risks of war. According to Western diplomatic sources, the NATO foreign ministers now meeting in Halifax, Nova Scotia, are distressed by Mr. Reagan's hardening position on the treaty.

Many members of Congress are also worried about the impact of abandoning the agreement. Discussion is already under way in Senate quarters about introducing a binding resolution to force the President to stay within the numerical limits of SALT II.

The President this week announced he has decided to disregard the SALT II limits at the end of the year unless the Soviets take measures to end alleged violations of that and other arms accords and negotiate seriously in Geneva.

But at the same time the President has ordered the dismantling of two Poseidon submarines, which keeps the United States within the treaty limits.

Diplomatic observers suggest this decision was made in order not to upset the prospects of another superpower summit.

``The President avoided the worst,'' says Dimitri K. Simes of the Carnegie Endowment for International Peace. ``If the subs were not dismantled, there would have been a rise in superpower polemics and a collapse of the arms control regime.''

Reagan has never liked SALT II, which he calls ``fatally flawed.'' But, with the support of the Joint Chiefs of Staff, the State Department, and the allies, he has pursued a policy of abiding by the agreement as long as the Soviet Union did likewise.

The dismantling of two Poseidon submarines will clear the way for the eighth Trident submarine, which joined the fleet this week. This keeps the number of US missiles with multiple warheads (MIRVs) within the 1,200 limit set by the SALT II pact.

But the President also says he is prepared to go beyond the limits as more B-52 bombers are equipped with air-launched cruise missiles (ALCMs). The treaty sets a ceiling of 1,320 on the combination of such bombers and MIRVed missiles. The US is close to the limit and due to exceed it by the end of the year when cruise-missile modifications are complete on the 131st B-52 bomber.

The President said he would reconsider his decision later this year if the Soviet Union takes ``constructive steps'' to alter the present arms control situation, including ending violations of arms agreements, moving forward in the Geneva arms talks, and slowing its military buildup.

Liberals and moderates within the arms control community are extremely disappointed. ``The administration has basically said it will back off arms control by the end of the year,'' says Jack Mendelsohn, an official of the Arms Control Association. ``That is bad news.''

But the President has left himself wiggle room. He stressed that the Poseidons are being retired not for arms control reasons but because of the enormous expense of refurbishing them.

So at year's end this economic rationale could be used to dismantle more Poseidons, instead of halting the fitting of B-52s with ALCMs, thus keeping the US within the SALT II constraints.

Diplomatic observers suggest that much will depend on whether the US-Soviet summit meeting takes place and what the results of the summit are.

In any case, the Soviets are not expected to meet the President's conditions for compliance with SALT II. The administration charges that Moscow is violating various arms treaties by (1) introducing two new types of land-based missiles, the SS-24 and SS-25; (2) encoding missile test-flight data needed to verify the treaties; (3) exceeding the bomber limits by failing to dismantle many of their bombers; and (4) building a ballistic-missile detection and tracking radar at Krasnoyarsk.

The alleged violations remain in dispute, however. For example, the Soviets claim that the SS-25 is a permitted modernization of the SS-13 missile.

In the matter of the Krasonyarsk radar, arms experts agree this appears to be a clear violation of the Antiballistic Missile Treaty of 1972.

Hard-liners within the Pentagon have consistently campaigned for abrogation of SALT II. But administration supporters of the pact, including many in the military, point out that if the treaty is scrapped, the Soviet Union is better positioned to add thousands of new warheads to its arsenal. Also, SALT II has forced the Soviets to dismantle hundreds of missile systems.

Some analysts see politics playing a role in the presidential announcement. They say Reagan has stepped up his rhetoric to placate the conservatives in his party as the 1986 election looms.