IF THE REAGAN administration means to go ahead with its Star Wars proposals, it ought to think twice about objecting to the Soviet Union's deployment of new radar facilities on the grounds that they violate the 1972 antiballistic missile (ABM) treaty.

The objections that the Reagan administration are raising have some validity, but a future president, wanting to move Star Wars along, may regret that Reagan pressed the complaints.

The 1972 treaty bars placing a particular type of large, sophisticated radars in the interior of a nation, prohibits introduction of smaller, mobile radars and limits to 100 the number of ABM launchers a nation can have around its one permitted ABM site. The Soviets, U.S. officials have said, are doing all those things in a program that could permit them to "break out" of the treaty limits in coming years and quickly move to a nationwide ABM system, something also barred by the treaty.

The treaty also, however, prohibits development, testing or deployment of space-based ABM components, devices that would be at the heart of President Reagan's proposed defensive system if research on it is successful over the next five to seven years.

Last month in Geneva, Secretary of State George P. Shultz told Soviet Foreign Minister Andrei A. Gromyko that some Soviet activities violate provisions of the 1972 ABM treaty. Underlying the Shultz complaint was the appearance in central Russia of a giant, almost-completed, phased-array radar, smaller transportable radars that have turned up in other areas and a new ground-to- air launcher system that could attack incoming missile warheads as well as aircraft.

But Shultz also carried another message to Gromyko in Geneva, one that the president repeated in his second inaugural address as he argued for defensive system against nuclear missiles. The goal of some future U.S. space-based defensive system, the president said, would be "a security shield that will destroy nuclear missiles before they reach their target."

Indeed, he said it was a moral issue for the United States to turn away from a doctrine that calls for deterring war by threatening to kill people with offensive nuclear weapons and substituting instead an effort to develop a system that "would render nuclear weapons obsolete."

How do we rationalize what the president has declared as the broad moral goal of promoting competition for a system that "wouldn't kill people; it would destroy weapons" with his administration's parallel campaign to halt what U.S. officials say is Moscow's present effort to develop such a system?

The first and basic answer is, as one administration official put it, that "it is illegal for them to do it," because the Soviets are acting in violation of a treaty that strictly forbids development of a nationwide ABM system. The president's Star Wars research, on the other hand, will not violate provisions of that treaty, U.S. officials have said, at least not in the next five years.

The 1972 treaty established a standing consultative commission based in Geneva to which both sides take their complaints about the other's alleged violations of the agreement. The Soviets to date have failed to satisfy U.S. arguments over ABM violations, calling the giant radar a space-tracking facility. On the other side, American representatives have similarly denied Soviet allegations that U.S. activities have violated testing prohibitions.

Shultz has said of his Geneva discussion that he wants "to reverse the erosion of the ABM treaty."

"We want to keep it pristine," one official said, "at least until sometime in the future" when it comes time to bend the treaty's language to fit whatever nationwide ABM system some future U.S. president may want to deploy.

Reagan and Shultz have both said that before any elements of such a future system could be tested or deployed -- steps now barred by that same ABM treaty -- the United States would discuss amending the language of the agreement to fit the proposed system. That moment, when laser devices or particle beam weapons have proven feasible on paper and are ready for the testing of a prototype, will come years after the Reagan administration has left office, in 1992 or even later.

And what will that future president do if the Soviet Union's leaders, looking back at what Reagan did to block their system in 1985, decide they will not negotiate changes to accommodate a space-based American ABM system?

The current American position that U.S. research for a future nationwide ABM system is permissible but Soviet development of present technology is a treaty violation, is far from the first philosophic inconsistency in the confused world of nuclear weapons.

It should, however, give Reagan administration officials enough pause to cause them to reconsider their approach. If defense is in fact a worthy, moral goal, it might be better to try to negotiate modifications of the present treaty to allow and even encourage both nations to step up the development of defensive systems.

The U.S. approach raises the question of whether the president believes the world could be made safer by both sides pursuing strategic defense rather than offense, or whether he thinks world safety depends on the United States developing a defense against Soviet missiles before Moscow develops one against American ICBMs.

Gromyko, in his unusual two-hour, televised press conference Jan. 13 mused on this point. He talked of Reagan's desire to "create a shield to protect them (the Americans) from . . . the Soviet Union" but was reassured that the United States "does not have the intention of striking a blow at the Soviet Union." Therefore, Gromyko said he was told, Moscow had no need for such an ABM system.

Gromyko then said he asked Shultz, "If we were to mentally trade places with you, the United States of America . . . If we were trying to create such (an ABM) system . . . would our corresponding statements (that the Soviets had no intention of attacking the United States) . . . be sufficient for you?"

The response, Gromyko said, was "silence, silence."