AFTER YEARS of harsh talk and mutual suspicion, the United States and the Soviet Union are closer to major progress on nuclear arms control today than at any time since 1979. It is now possible to outline a series of far-reaching agreements which, with sufficient political will on the part of both sides, can be achieved in the next few years.

In fact, the two sides are closer on these matters than anyone has yet conceded. If the final gaps can be bridged, mutually verifiable agreements can be reached to achieve the following three goals:

*The removal of all American and Soviet intermediate range nuclear forces from Europe;

*A halt to all future testing of nuclear weapons; and

*A drastic reduction in multiple-warhead, heavy land-based missiles. This would be linked to a ban on the development and deployment of space-based anti-ballistic missile defense systems, while permitting research on such systems.

My belief that such progress can be made is based on my experience as a member of the Senate Observer Group, which has been monitoring the arms talks in Geneva for the past year, and on my lengthy meetings earlier this month with General Secretary Mikhail Gorbachev and Foreign Minister Eduard Shevardnadze of the Soviet Union.

In his most recent public proposal, General Secretary Gorbachev announced the Soviet Union's willingness to withdraw all of its SS-20s from Europe, if the United States withdraws its Pershing II and cruise missiles from Europe, and if Great Britain and France agree to freeze their nuclear missile systems.

The Gorbachev offer appeared to be a partial acceptance of President Reagan's "zero-zero" proposal, which would have banned all U.S. and Soviet intermediate-range nuclear forces (INF), wherever based. It was initially part of a larger, three-phase package ostensibly aimed at eliminating nuclear weapons altogether. At the time, the central problem with Gorbachev's INF offer was that Soviet acceptance of the U.S. position appeared to be linked to a Soviet demand that the U.S. must abandon President Reagan's Strategic Defense Initiative (SDI), the so-called "Star Wars" proposal.

During my conversation this month with Gorbachev, he told me explicitly and unequivocally that there are no "preconditions" to negotiating the immediate removal of Soviet and American medium-range missiles from Europe. These negotiations can be successful even if there is no progress in the SDI talks. Gorbachev knows he is making a concession on this point, and he expressed the hope that it would make quick progress possible.

There are, of course, other important issues to be resolved before an INF agreement can be signed. Will modernization of British and French nuclear forces be permitted? How many SS-20s can be deployed in Central and Eastern Asia? Will the Soviets accept the verification measures that we will require? These are difficult questions, but solutions exist.

For example, the 1983 "Walk in the Woods" formula suggested by Ambassador Paul Nitze would have required the Soviet Union to freeze its SS-20 missile launchers in Europe at 90. As part of a new INF agreement, the U.S.S.R. could reduce its launchers to that level.

The issue of British and French forces will be more difficult. Yet a middle ground is visible in which British and French nuclear modernization and replacement could go forward -- as long as the number of warheads is not substantially increased after U.S. and Soviet INF reductions go into effect.

A breakthrough in INF has now occurred, and solutions to these other issues should be immediately explored in Geneva. Gorbachev views INF forces as the most dangerous nuclear flashpoint between our nations, since the missile flight time is so short -- under 10 minutes. Removing this flashpoint would be a major step toward arms control.

In our conversation, Gorbachev also emphasized that the Soviet Union is prepared to enter into an agreement banning all future nuclear testing, without any precondition or requirement of progress in other areas. In fact, he expressed the hope that the current unilateral Soviet moratorium would be matched by the United States, and that his country would never again have to conduct a nuclear test. Gorbachev and Foreign Minister Shevardnadze -- with whom I had a separate meeting -- both stressed that they are willing to agree to a range of measures, including on-site inspections, to verify compliance.

Nuclear testing in the atmosphere has been prohibited since the Limited Test Ban Treaty of 1963, negotiated by President Kennedy and Nikita Khrushchev. Today, the Reagan administration opposes a comprehensive test ban, because it feels that continued underground testing is essential to maintain the reliability of our nuclear stockpile.

Even if the administration does not accept a moratorium, a compromise is still possible here. The solution involves three steps: first, to ratify the Threshold Test Ban Treaty and the Peaceful Nuclear Explosions Treaty; second, to improve verification measures to monitor testing; and third, to phase in lower and lower thresholds on future tests.

The 1974 Threshold Test Ban Treaty limits U.S. and Soviet underground test explosions to a yield of less than 150 kilotons. The 1976 Peaceful Nuclear Explosions Treaty provides comparable limits on nuclear tests for peaceful purposes. These two treaties were signed but never ratified by the Senate. Both sides continue to observe them, but the Reagan administration has refused to endorse them until the Soviets accept more extensive measures for verification; by a vote of 77-22 in 1984, the Senate urged the administration to support the treaties.

The Soviet leaders told me that they would agree to additional verification measures only in the context of a total ban on testing. They stressed that the two existing treaties contain verification provisions that have not been put into effect because the United States has not ratified the treaties.

To bridge this difference, the two sides should permit technical teams to visit each other's test sites to observe tests and to calibrate monitoring equipment needed for accurate measurement of yields. With the assurance gained from this step and other verification measures, the 150-kiloton threshold could be reduced over time, allowing for high-confidence verification and moving toward a final, total ban on all nuclear tests.

A comprehensive test ban would be an extremely effective step toward halting the nuclear arms race. There are other ways -- short of underground testing -- to guarantee the reliability of our nuclear arsenal. By ratifying the two pending treaties, by improving verification, and by gradually lowering the thresholds for future testing, both nations will build confidence in the possibility of verifying a comprehensive ban; in the interim, the Reagan administration will be able to conduct the low-yield tests which they say are currently needed. However, as testing thresholds decline, some current high-yield testing will be prohibited, including tests of the nuclear-pumped X-ray laser now being conducted under the SDI program.

The central arms-control issue today is the question of long-range offensive and defensive forces; neither an INF agreement nor a comprehensive test ban can substitute for achieving a breakthrough in this third, most critical area.

To date, the debate on the issue of strategic defenses has been largely rhetorical and unproductive. Reagan insists that SDI is primarily a research program to determine whether space, air, and ground-based antimissile systems can someday be combined to establish a leak-proof shield that would render nuclear missiles obsolete. Gorbachev reiterated to me his familiar position that SDI is aimed at developing "space-strike weapons" that will ultimately enhance, not replace, U.S. offensive missile systems. Gorbachev believes that the United States is seeking nuclear superiority over the Soviet Union, in order to threaten a first strike with its "sword" of offensive missiles and then use its "shield" of SDI to neutralize Soviet retaliation.

The Soviets have now made clear that they are willing to accept drastic reductions in their long-range warheads in exchange for limiting our SDI to "basic" research. The real challenge is to draw the line between acceptable research -- which would allow President Reagan to explore the feasibility of his dream for a space shield -- and unacceptable weapons development, which would violate the ABM Treaty and launch a new and dangerously destabilizing escalation of the nuclear arms race.

I posed this question to Secretary Gorbachev: Is there a way to narrow the difference between Reagan's insistence on SDI research and Soviet fears that such a program will lead to "space-strike" weapons? I told Gorbachev that there may well be room for agreement if the two sides abandon their confrontational rhetoric and begin a practical discussion of possible areas of compromise. Resolving this issue would pave the way for substantial reductions in the strategic offensive forces of both sides. But I sensed little or no Soviet flexibility on this issue.

Gorbachev was clearly incensed by the Reagan administration's recent attempt to reinterpret the ABM treaty to ease its strict prohibitions against antimissile systems. Criticizing the administration's search for loopholes in the treaty, Gorbachev called the new U.S. interpretation the work of a "legal expert on pornography."

I pointed out that negotiating a compromise on Star Wars will require better teamwork between the U.S. and Soviet scientists who understand the nature of the research programs being conducted by both sides, and our most skilled negotiators, who will have to craft acceptable treaty language based on the technical possibilities. In my view, these detailed discussions should begin at once in Geneva.

If such a compromise can be negotiated, it will then be possible to reach an agreement that includes sharp reductions in the Soviet multiple-warhead heavy missile force (SS-18s) and a halt in the deployment of MX missiles by the United States. A vital complement to such reductions would be a statement of objectives for further agreements -- such as a ban on all new MIRVed ICBMS, mobile or fixed-based, and the deployment of a mutually verifiable number of single-warhead, mobile missiles.

I came away from my meeting with Gorbachev convinced that he is a strong leader whose goal is improved efficiency of the Soviet system, rather than fundamental reform. His world view is of a continuing struggle with the West, and he believes in wars of national liberation. On the issue of human rights in the Soviet Union, he adheres rigidly to the hard-line Soviet position, and progress beyond the recent gestures is likely to be painfully slow.

In sum, there are many areas where our two countries will continue to be at odds, and major tensions will remain. But I believe Gorbachev shares Reagan's view that a nuclear war cannot be won and must never be fought. On that fundamental question, the interests of the Soviet Union and the United States coincide, and important steps are possible in the coming months toward a real and lasting nuclear peace.

Reagan and Gorbachev have an historic opportunity to pursue their common goal of a world without nuclear weapons. With that shared vision, and with our shared security interests, I believe the next steps on arms control are there for the taking.