The quagmire that President Carter is about to enter in the battle of SALT II is depicted by a secret State Department working paper that contradicts both publicly stated policy and expert opinion.

The working paper asserts that the Minuteman, the U.S. intercontinental ballistic missile (ICMB), can survive. Soviet attack through the 1980s if a hotly opposed protocol to SALT II is extended. Yet Carter administration officials are selling the prospective new treaty on grounds that the protocol need not be extended when it expires in 1980. Moreover, Pentagon experts say there is no way SALT II - protocol or no protocl - can protect the Minuteman from Soviet attack.

Those contradictions pose the central question about the new strategic arms limitation treaty nearing completion: Would the United States be less or more secure because of SALT II? The intrigue that lies ahead in debating that question may well make the president look back at his just-completed Panama Canal agony with nostalgic pleasure.

A key figure is Sen. Henry M. Jackson (D-Wash.). Without his support, Carter will find it difficult if not impossible to win the two-thirds Senate vote for ratification. Jackson privately says SALT II so preserves and augments Soviet stragetic advantages that he cannot back it.

Yet White House strategists find a ray of hope toward winning over Jackson and other defense-oriented senators. Jackson finds nothing all that onerous in the treaty itself, which would expire in 1985. He objects to the protocol, expiring in 1980, which prohibits mobile missiles (as well as limiting cruise missiles).

The restriction on mobile missiles becomes decisive because recent Soviet missile testreveal a dramatic break-through in accuracy of Soviet SS18 and SS19 ICBMs. On the basis of that break-through, some Pentagon analysts now estimate that the Minuteman would be vulnerable to Soviet attack by the early 1980s. Put in plain language, that means a major element in U.S. strategic deterrence will be unable to survive a Soviet attack as early as three years from now.

The antidote is to change the present Minuteman solos and develop mobile missiles (pushing ahead the now-suspended MX survivable ICBM basing system). But all such activity would be prohibited by SALT II, which does permit the Soviet Union to transform single-warhead missiles into hundreds of multiple-warheads and thereby threaten the Minuteman.

The answer considered by White House strategists is to make clear that the MX is prohibited only until the protoccl expires (in 1980 by U.S. calculations, later by Russian). The Scoop Jacksons can be reassured that MX development will begin then.

Thus the State Department SALT working paper (dated March 7 and titled "The Viability of the U.S. Strategic Deterrence") comes like a warning signal in the night: "The constraints on qualitive improvements to the Soviet ICBMs in the draft SALT protocol have the potential to improve the Minuteman survivability problem as long as the protocol is extended beyond 1980 . . . We estimate that extending the protocol might increase the theoretical, number of Minuteman survivors in 1966 from about 100 to about 200."

The claim that SALT II can save the Minuteman echoes two secret studies prepared for the president by academically oriented theoreticians. A White House-contracted study (called the Press Report after presidential science adviser Frank Press) denied any "imminent" threat to the Minuteman, which "will remain a significant survivable component of the U.S. deterrent throughout most of the 1980s." A Defense Department-contracted study (called the Jason Report) is even more blunt: "Present Soviet strategic forces pose no threat to the Minuteman."

The response by Pentagon experts has been sheer outrage. Internal memoranda at the Defense Department contend the two studies reach "unsubstantiated, opinionated and unfounded conclusions" and "indulge in propaganda-like statements." Defense Department officials have not minced words in closed-door testimony on Capitol Hill: The Minuteman is quickly becoming vulnerable.

But is the president getting the word? Pentagon experts doubt it. And if Carter believes the Minuteman is truly protected under the protocol of SALT II, he becomes entwined in a mass of contradictions from which there is no easy escape.

If the Minuteman can survive through the 1980s, why do the Pentagon experts say otherwise? If it cannot, will Carter promise development of mobile missiles once the protocol expires? If he does, what about State Department claims that the protocol protects the Minuteman? How, then, can Jackson's concerns about Soviet strategic advantages be satisfied consistent with pushing SALT IT? Jimmy Carter may yet yearn for the classic simplicity of wrestling with the DeConcini reservation.