Presidential spokesman Larry Speakes said yesterday the newest Soviet arms proposal "has merit" and a White House official said later the Soviet offer is "not being taken as a propagandistic endeavor."

The remarks by Speakes and other White House officials were intended, sources said, as a positive signal to the Soviets on the eve of President Reagan's address on arms control and U.S.-Soviet relations today in Glassboro, N.J., site of the 1967 meeting between President Lyndon B. Johnson and Soviet Premier Alexei Kosygin.

A Soviet diplomat said yesterday that Moscow is looking for a positive response to its latest offer as a step toward arranging the delayed summit-planning meeting between Secretary of State George P. Shultz and Soviet Foreign Minister Eduard Shevardnadze.

The new administration effort to send positive messages to the Soviets follows a series of crossed signals between Washington and Moscow over both the summit and the Geneva arms control talks. Diplomacy was complicated by the controversial White House decision to abandon the SALT II limits on strategic weapons, announced May 27.

A Soviet diplomat disclosed yesterday that the SALT II decision came at a moment when officials in Moscow were contemplating how to respond to a letter from Reagan to Gorbachev received days earlier that was intended to reinvigorate the summit-planning process.

The administration has also come under intensifying pressure from Capitol Hill to abide by the SALT II limits, to restrict the president's Strategic Defense Initiative by sharply curtailing its funding and to respond constructively to new Soviet arms control proposals seen by some members of Congress as relatively forthcoming.

Reagan's address today is to be "a signal of his positive approach to the superpower relationship and his readiness to engage in top-level dialogue," the White House official said.

In his May letter to Gorbachev, according to U.S. and Soviet officials, Reagan suggested a meeting of Shultz and Shevardnadze in Europe to begin preparatory work for a summit.

The Soviet diplomat said Moscow dropped any idea of responding directly to Reagan's letter once the SALT II announcement had been made. But the Soviets decided, he said, to go ahead with their planned presentation in the Geneva arms talks of the new proposals on strategic and space defense weapons that had already been prepared. "That was our response," he said.

Yesterday, a State Department official said the president's letter was sent "perhaps a week before the decision" on SALT II but insisted "there was no link between the letter and Reagan's decision" on the treaty.

The Soviet offer, which linked U.S. adherance to the 1972 Anti-Ballistic Missile treaty to reductions in strategic weapons, initially drew a critical reaction from Defense Secretary Caspar W. Weinberger. But then the administration clamped down on comments from officials and said only that it was under serious review.

Yesterday's statements appeared to be the first clear sign that the administration saw some movement in the Soviet proposal from previous ones.

The Soviet diplomat said yesterday that Moscow has moved more than halfway in several areas that had been described as unacceptable by the U.S. in earlier offers. He cited, as one example, that Moscow no longer demanded inclusion of some European-based, nuclear-capable fighter planes as strategic weapons.

A White House official said yesterday described this Soviet move as "a concession," but said other aspects of the proposal dealing with space defenses had many more unanswered questions.

The new Soviet ambassador, Yuri V. Dubinin, is expected to return to Washington from Moscow tomorrow and present his credentials to Reagan on Monday. His meeting with Reagan, coming after the Glassboro speech, could offer another opportunity for the two sides to discuss the summit and arms control.