NEWS ANALYSIS : Sides Decide to Split the Difference : Arms control: Two superpowers adopt a spirit of compromise to break impasses and ensure a successful summit.

It took an extra day of sometimes rancorous negotiations, but the United States and the Soviet Union have broken the back of a vexing disagreement that could have dimmed the summit meeting between President Bush and Soviet President Mikhail S. Gorbachev beginning May 30.

The compromises on seemingly abstract range and counting rules for cruise missiles provide the basis for an arms control agreement that will permit Bush and Gorbachev to claim success for their Washington meeting. With anything less, the summit probably would be counted a failure.

Secretary of State James A. Baker III and Soviet Foreign Minister Eduard A. Shevardnadze said that a few details remain to be worked out. But both men predicted that the remaining issues could be settled before Gorbachev arrives in Washington.

According to senior U.S. officials, the deal was struck Friday during a five-hour meeting between Baker and Gorbachev. But the agreement almost fell apart a few hours later when Soviet military leaders tried to impose new conditions and pull back from concessions Gorbachev had accepted.

In the end, the senior official said, the Soviet side agreed to return to the Baker-Gorbachev understanding.

On most matters, the two sides simply split the difference between the official positions.

But Gorbachev accepted the U.S. demand that the only limitation on sea-launched cruise missiles would be “politically binding” statements of intent by both sides, without any requirement for verification.

Washington opposes verification of limits on sea-launched cruise missiles because nuclear missiles--which would be covered by a strategic arms treaty--cannot be distinguished from conventionally armed missiles, which would not.

The Soviet side agreed to such an unverified statement last February but then pulled back. In effect, the Soviets have returned to their earlier concession.

In exchange, the United States agreed to a limit of 880 on sea-launched cruise missiles. Washington had hoped to avoid any numerical ceiling, but the 880 figure is at least 100 higher than the number the Pentagon plans to buy.

The United States also agreed to exclude sea-launched cruise missiles with ranges of less than 375 miles. Washington had wanted to include weapons with ranges of more than 185 miles. Officials explained that the Soviet Union has a number of aging missile systems with ranges between 185 and 375 miles. Under the compromise, these will not be restricted.

On air-launched cruise missiles, the United States agreed to a Soviet demand that all missiles with a range of more than 375 miles be counted by the treaty after the Soviet side agreed that a new American missile known as Tacit Rainbow would not be included.

The Tacit Rainbow project is intended to be armed with non-nuclear warheads, which normally would mean it would not be included in the treaty. However, under a provision that was accepted months ago, all cruise missiles that had been tested by the end of last year--including Tacit Rainbow--would be presumed to be nuclear. The compromise changed the effective date for the earlier agreement to the end of 1988, thus exempting Tacit Rainbow from the limitations.

According to U.S. officials, Gorbachev and Baker agreed to the compromise language for both classes of cruise missiles. However, they said, lower-ranking Soviet officials, particularly those from the military, tried to impose new conditions on Tacit Rainbow, such as banning modernization of the missile, restricting its range and imposing a guarantee that it would never have a nuclear warhead. All of those restrictions were unacceptable to the United States.

“In the Gorbachev meeting, we did feel that we had broken the back of the issues,” one senior official said. “The package that we thought had been put together began to unravel because they pulled a piece out of it.”

Another official said that arms control specialists wrestled with the issue all morning Saturday. About noon, he said, Baker sent a letter to Shevardnadze saying that, if the Soviet side continued to push additional restrictions, there would be no deal. Shevardnadze accepted the U.S. position a few hours later.

The arguments seem technical and abstruse, but just such problems have blocked all previous attempts to control cruise missile technology.

The latest U.S.-Soviet compromises probably would not have been possible without the overall warming in superpower relations. As an example of the changed mood, a senior official said that, late in the Friday meeting, Gorbachev broke one deadlock by asking Baker: “How about if we split the difference?”

“Baker said, ‘You’ve got a deal.’ And they reached across the table and shook hands.”