Last 15 minutes in Geneva

Soviet Foreign Minister Andrei Gromyko once said that the last 15 minutes were the critical ones in any negotiation. Only then do the two sides finally expose their real positions and not just their bargaining positions.

The United States and the Soviet Union are rapidly approaching ''the last 15 minutes'' in their medium-range missile negotiations in Geneva. There will be success or failure between early September, when the talks resume, and mid-December, when the US will take an irreversible step and begin deployment of 572 Pershing II and ground-launched cruise missiles in Western Europe.

The US needs a new approach to the negotiations before they resume. The present approach - rejected by the Soviets - is too one-sided either to test whether the Soviets have a genuine interest in an agreement or, if negotiations fail, to broaden support in Western Europe for the new American deployments.

At first sight, the administration's latest proposal appears reasonable and flexible. The US will accept equal levels of warheads on land-based missiles starting with 50 warheads and going up to 450 on each side. This offer seems fair and equitable, but is it really?

The administration's proposal limits only medium-range missiles based on land. The US has none. In contrast, nearly all the Soviet medium-range missile force is based on land. Western missiles with an intermediate range - British and French - are based mainly at sea. Thus the US proposal would reduce the missiles that threaten the West but not those that threaten the East. Not surprisingly, the Soviet negotiators have rejected this proposal. They insist that British and French missiles be taken into account.

Moscow is unlikely to capitulate on this issue. It is faced with a qualitative as well as quantitative expansion in American, British, and French forces. With the deployment of 572 Pershing II and ground-launched cruise missiles, the US will for the first time since 1963 be able to strike the Soviet homeland with missiles from Western Europe. With the purchase of the highly accurate Trident II missiles from the US, the British will for the first time deploy missiles capable of destroying Soviet ''hard targets.''

Moreover, both the British and French plan to replace their single-warhead missiles with multiple-warhead missiles; numbers of British warhead targets may expand eight-fold or more, and French numbers may expand sixfold. By Soviet estimates, the number of British and French warheads will jump from 300 to more than 1,300. The West would then have about 1,900 warheads, almost twice the number of Soviet warheads now aimed at Europe. The Soviets may have a compelling interest in preventing this threatened expansion of Western forces.

The administration maintains that the Soviets will not be seriously interested in a reduction agreement until after the US has begun deploying cruise and Pershing II missiles in December 1983. But Moscow says that more Western arms will result in more Soviet arms - not in concessions at Geneva. The Soviets are specific about new deployments in Eastern Europe. When the US began deploying multiple independently targetable re-entry vehicles (MIRVs) on its missiles in 1969, the Soviets responded not with arms control concessions but by developing and deploying their own MIRVed missiles. The MIRV experience is being repeated with cruise missiles, and the Soviets are moving fast to close this gap.

The potential deployment of arms - arms not yet deployed like Pershing II and ground-launched cruise missiles - provides leverage and preserves negotiating flexibility to achieve arms reduction. But the actual deployment of more arms has the opposite effect and evokes an arms response. American bargaining leverage in Geneva is at its maximum between now and December, before new American deployments have actually begun and before we have taken any irreversible steps.

When the negotiations resume in September, the US should propose to the Soviets the restoration of the overall warhead balance on medium-range missiles that existed prior to the significant expansion of Soviet warheads, which began in 1977 with the deployment of the multiple-warhead SS-20. This proposal would involve three undertakings:

First, the Soviet Union would dismantle missiles carrying about 700 warheads (about 243 are on obsolete SS-4s and 5s) leaving about 600 Soviet warheads on medium-range missiles, the number deployed before 1977. British and French forces would not be taken into account in calculating these reductions. This would meet the Western position. On the other hand, if Moscow retained 300 warheads targeted on Europe and the balance targeted on Asia, the outcome from the Soviet point of view would be the same as if the reductions were based on the British and French level.

Second, the US would cancel the planned deployment of the 572 Pershing II and ground-launched cruise missiles. The substantial Soviet reductions would eliminate the justification for these new American deployments.

Third, the US and the Soviet Union would have the right to compensation if additional medium-range missile warhead deployments were made by any member of the Warsaw Pact or NATO. Because of the significant lessening of the threat from the Soviet Union, Britain and France might defer their planned increases. On the other hand, this compensation provision would ensure that the Soviet Union could match any British and French increases, should they occur.

If the Soviets agreed to this proposal, the nuclear arms race in Europe would be halted. Europe would be more secure.