The shadow of Jesse Helms over President Reagan's arms control policy was privately revealed when a State Department cable sent to U.S. allies inserted this sentence in the version publicly released in the United States: "No decision has yet been taken on our adherence to existing SALT agreements."

The March 4 cable was dispatched to contradict Secretary of the Navy John Lehman, who had announced there was no legal basis for the United States to continue honoring SALT I or SALT II. It said the United States would not "undercut existing agreements" so long as the Soviet Union did not either.

That cable, representing Secretary of State Alexander Haig's policy, was designed to placate NATO members, especially touchy West Germany. The domestic version, with the "no decision" sentence, was designed to placate right-wing Republicans led by Sen. Helms, who worry that Reagan's SALT policy has not yet taken a decisive turn away from Jimmy Carter's. Which version will be Ronald Reagan's ultimate policy has triggered an intense power struggle in Washington.

That struggle is Helms vs. Haig, but its roots are buried all through the capital. Haig's State Department and the Central Intelligence Agency are in combat against top officials of the Arms Control and Disarmament Agency. The ACDA officials, carryovers from the transition, are closely tied to Helm's conservative Republican bloc in the Senate but may soon be replaced by permanent arms control officials in tune with Haig.

Helms is plotting a counterattack against Haig's position on sticking to SALT I and the unratified SALT II for now, with ramifications that will compound Haig's problems with the European allies. If he proves that the Soviets have systematically violated SALT agreements, Helms could force Reagan into a confrontation not only with Moscow but also with Bonn.

Helms opened early this month with a letter to the hard-line, acting director of ACDA, Michael Pillsbury, asking for a complete report on one of the most sensitive SALT issues: verification by the United States of Soviet compliance -- or Soviet cheating.

Coincidentally, a Pillsbury deputy. ACDA counselor David Sullivan, is putting finishing touches on a study that documents 23 alleged SALT violations by the Russians. Although ex-CIA analyst Sullivan is known as a hardline, anti-Soviet ideologue, no serious student of SALT doubts his competence. His study in the hands of Helms and other anti-SALT Republican senators could close off Reagan's SALT options.

In fact, Pillsbury laid out the argument for Soviet cheating in his April 14 reply to Helms: "We are unable to reassure you that these agreements are adequately verifiable at the present time." An amendment to the law that established ACDA asserts that "adequate verification of compliance should be an indispensable part of any international arms control agreement." It orders the ACDA director to so "report" to Congress.

Even before Helms' request was sent to Pillsbury, the CIA was harassing Sullivan's study of Soviet violations. Reagan's appointed CIA director, William Casey, has not shown his hand. But at CIA's working-group level staffed by holdovers from the Carter administration, the effort to silence Sullivan is intense They reflect the CIA's habitual caution in dealing with the touchy violations issue.

The clock is running against the hard-liners Reagan is ready to nominate an establishment conservative Democrat, law professor Eugene Rostow of Yale, to be permanent ACDA director. It is not even certain that he, Sullivan and other hard-liners will remain in the agency.

But nobody can make the clock run more slowly than Jesse Helms. He will put a hold on Rostow's nomination just as he has on seven other presidential nominations (most of them Haig's recommendations) to State Department positions, paralyzing the department's operations. The source of all this is the unrequited expectations of Helms and other conservative arising from the president's campaign oratory.

Helms' move against Haig's deliberately ambiguous, cautious policy not to undercut "existing agreements" with the Russians is based or Reagan's campaign description of SALT II as "fatally flawed." The conservatives who helped elect Reagan have waited three months into his presidency for action to fit those words.

So far, Reagan has listened to Haig's advice: If the United States tells the allies prematurely what it really thinks about SALT I and SALT II, the alliance could be imperiled. Whether the president sticks with Haig's advice much longer is now in question, thanks to the formidable resources of Helms and his allies.