Prospects brighten for superpower arms accord. But Reagan urges Soviet changes on host of issues

President Reagan is telling the Soviet Union that East-West relations could be on the verge of a historic improvement - if Kremlin leaders make significant moves to show their good faith. That message emerged from the President's speech Wednesday to the Town Hall of California, which took place amid indications that one of the actions called for, a superpower agreement to eliminate medium-range nuclear weapons, may soon come to pass.

``Is far-reaching, enduring change in the postwar standoff now possible?'' Mr. Reagan asked in his speech.

The opportunity for such change is too great to let slip by, the President said. He cited as positive the move toward greater openness in Soviet life under Mikhail Gorbachev, and Soviet-American cooperation in attempts to end the Iran-Iraq war.

He challenged the Soviets to ``show us and the world their seriousness about fundamental improvements'' by meeting United States-designated ``guideposts.'' The list was mainly a repeat of past Reagan proposals. It mentioned a number of actions that would be revolutionary if carried out. Among them were destruction of the Berlin Wall, political self-determination for the Soviet satellite nations of Eastern Europe, Soviet withdrawal from Afghanistan, and joint talks on US-Soviet cooperation in erecting defenses against ballistic missiles.

``Steps such as these are required for a fundamental improvement in relations,'' said Reagan.

The President also called for a number of secondary actions. Among them were publication by the Kremlin of details about the cost and size of Soviet armed forces, open debate in the Supreme Soviet on matters of military policy, and agreement with the US on an intermediate-range missile pact.

The speech was written with two audiences in mind, suggested a White House official interviewed before the talk was delivered: Soviet officials and the American public.

``You're telling both we want better superpower relations, but we're not going to indulge in any self-delusions about the nature of the Soviet Union,'' the official said.

Meanwhile, halfway around the world, the pace of negotiations quickened on a treaty that would ban medium-range nuclear missiles. There were hints of a possible East-West compromise on the issue of West German Pershing 1A missiles, and US negotiators in Geneva submitted a new and less stringent plan for verification of a medium-range missile pact.

Administration officials said the plan wasn't a rollback in the US position. They said they have long told the Soviets that simpler verification would be possible if the Soviet Union dropped its insistence on retaining 100 medium-range missiles and agreed to eliminate all such weapons. Soviet negotiators have now accepted this so-called ``zero option.''

The US proposal would still represent ``the most stringent verification of any arms control agreement in history,'' said State Department spokeswoman Phyllis Oakley.

Specifically, the plan called for less intrusive measures in:

On-site inspections. Previously, US negotiators had proposed what they called ``perimeter-portal monitoring,'' in which an arrangement of cameras and human inspectors would be permanently stationed outside certain missile plants. The new plan drops this provision.

Challenge inspections. The old US negotiating position had essentially been that any facility, anywhere, could be subject to a short-notice challenge inspection. The new plan, however, would sharply limit these visits.

Medium-range-missile sites could be the subject of such visits, until the agreed-upon period for elimination of the missiles has passed. After that, only a few facilities in the US and the Soviet Union that meet still-to-be-negotiated criteria would be subject to short-notice inspection.

US officials feel the new verification plan will be easier for the Soviets to accept than the old. After resisting the concept of on-site inspection for years, the Soviets lately have accepted that such verification could be necessary, and they may not accept this change in plan with open arms.