Reiterating that his Strategic Defense Initiative (SDI) is "not a bargaining chip," President Reagan told 250 Republican leaders yesterday that it represented "the essence of science and spirit joining for mankind's highest ideal -- peace on Earth."

"It must go forward," Reagan said. "It will go forward. It is not a bargaining chip. And we will go forward."

Reagan's statement appeared to be partly a rebuttal to suggestions made anonymously by some aides and advisers that deployment of SDI could be traded for cuts in Soviet offensive missiles.

The Soviets have proposed a 50 percent reduction in some types of nuclear warheads of both superpowers in return for "cessation of work" on SDI.

But none of these officials used the phrase "bargaining chip." One said there was "running room" in the president's position, and several said there was room for compromise because Reagan has never defined the limits of SDI.

Soviet leader Mikhail Gorbachev, who has called for an end to SDI, has repeatedly acknowledged that there is room for "fundamental research" on strategic defense. Reagan usually calls for "research and testing" and his national security affairs adviser, Robert C. McFarlane, contends that only deployment of a missile defense, if based on new and exotic technology, is prohibited by the 1972 ABM treaty.

Yesterday, in defending his proposal, Reagan did not refer to "testing," saying instead that he was "determined to pursue our research program to explore the feasibility of strategic defenses -- a security shield that could protect the United States and our allies from missile attack."

White House spokesman Larry Speakes said that there was no significance to Reagan's omission of the reference to testing.

In a speech in the East Room, the president said SDI "would not kill people, it would destroy weapons . . . it would not militarize space, it would help demilitarize the arsenals of Earth."

He also joked about the news media attention paid to Gorbachev, saying that "after Time magazine I'm going to wear my pin-striped suit to Geneva." Gorbachev was interviewed last month by Time, which devoted its cover story to him.

Reagan's remark, delivered with a smile, drew laughter from his partisan audience. But White House officials have in a serious vein been drawing attention to the coverage received by Gorbachev, contending that the new Soviet leader follows policies indistinguishable from his predecessors despite his stylish dress and mastery of modern public-relations techniques.

Meanwhile yesterday, NATO Secretary General Lord Carrington said that Reagan and Gorbachev may be able to "give a push in the right direction" at next month's summit meeting toward a superpower agreement on reduction of nuclear arms.

Following meetings with Reagan and with Secretary of State George P. Shultz, Carrington described the Soviet Union's recent arms offer at Geneva as "a step forward" in what is likely to be a protracted process of "give and take" with the United States.

"If there really is a genuine wish on the Soviet part to negotiate," Carrington said, the U.S. and Soviet leaders may be able at their Nov. 19-20 summit to give a push toward agreement.

Carrington, who was British foreign secretary before assuming the NATO post last year, said the recent Soviet proposals are not acceptable to the United States and the rest of the West "as they stand." He added, "I don't suppose for one moment that the Soviets thought they would be accepted out of hand."

The NATO political chief seemed relatively unconcerned that the Soviet arms proposals, including new positions on intermediate-range missiles in Europe, could split the Atlantic alliance. "It's going to be extremely difficult to divide Europe from the United States and split NATO," Carrington said.

He described Gorbachev's statements in Paris about reductions in the Soviet force of SS20 intermediate-range missiles "on standby alert" in Europe as a "vague generality" that is unlikely to affect the decision of the Netherlands' government later this year about deploying U.S. ground-launched cruise missiles.

Regarding Reagan's Strategic Defense Initiative, or "Star Wars" research plan to defend against nuclear missiles, Carrington said that except for the French government, "I don't think there is a difference of opinion" between Western Europe and the United States. It is clear, he said, that "research" cannot be banned and that Reagan is willing to discuss with the Soviets what should be done in "later stages" of the program, such as testing.