On the face of it, President Reagan has struck a hard and seemingly nonnegotiable position on behalf of the Strategic Defense Initiative (SDI), his hope of finding an effective shield in space against Soviet nuclear missiles.

In speeches and news conferences Reagan has pledged unwavering determination not to yield on "Star Wars" when he meets Soviet leader Mikhail Gorbachev at their Geneva summit Nov. 19-20. On Friday, at a political speech in New Jersey, he said it again: "We will go forward with seeing if it cannot be made into a great protector of our people and the people of the world." But presidential aides and advisers who are involved in summit preparations suggest that Reagan is more flexible about Star Wars than he seems and more interested in reaching an accommodation with the Soviets than his rhetoric indicates.

"He has left running room," a senior White House official said last week, describing what he perceives as his boss' intentions, even if they are not fully consistent with his public statements.

Some aides say that Reagan has laid down "a hard marker" for the Soviets to let them know he would never give ground on SDI unless they make truly substantial cuts in their heavy, land-based intercontinental ballistic missiles. One Reagan associate familiar with the way Reagan bargains said the president will demonstrate that he is "a tough negotiator" but recognizes that the aim of any negotiation is to strike an agreement.

And one administration official suggested that swapping Star Wars for really deep cuts in Soviet missiles would be akin to "a sting" operation -- a trade of a visionary idea that many consider impractical for tangible reductions of existing and threatening weapons.

On the other hand, the president's aides and associates are avoiding predictions that Reagan and Gorbachev will strike a bargain in Geneva. They acknowledge how difficult that will be, given the suspicions on both sides and Reagan's genuine enthusiasm for SDI.

Reagan, who was president of the Screen Actors Guild in the late '40s and early '50s and led the union in its first successful strike, takes negotiations seriously. He has reminisced that he once settled a contract for the guild during a chance meeting in the washroom with the negotiator for the other side.

"You don't see a lot of pride expressed by Ronald Reagan, but in this area he takes pride," then-deputy chief of staff Michael K. Deaver said in 1983. "He thinks of himself as a negotiator, and he knows when to compromise."

Throughout his career Reagan has followed a set of basic principles in negotiating. He takes a firm opening stand, refuses to make what he calls "preemptive concessions," keeps his cards close to his vest and strikes a bargain only when the negotiation is in danger of being lost. Typically, he also uses some of his most inflexible rhetoric just before he strikes a deal.

As governor of California, Reagan said his feet were "set in concrete" against state income tax withholding, then approved it and blandly told reporters, "The sound you hear is the concrete cracking around my feet."

During the first two years of his presidency, Reagan vowed that he would keep the Social Security system intact. But in 1983 his top aides took the lead in negotiating money-saving changes in the system when Social Security faced a financial crisis, and Reagan took credit for having saved the system.

In 1982, a year after he had pushed major income tax cuts through the Congress, Reagan agreed to a "corrective" measure that raised $100 billion in new revenue. And in September, less than a month after declaring his opposition to "punitive sanctions" against South Africa, the president headed off congressional action by imposing sanctions recommended by national security affairs adviser Robert C. McFarlane and chief of staff Donald T. Regan.

All of these negotiations demonstrated different levels of Reagan's involvement and awareness. On the Social Security issue he was a canny negotiator who refused to sign off on any deal before House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) had done so first.

On the tax bill, Reagan let aides convince him that the bill was really a "tax reform" that corrected "unintended loophooles" of the previous year's bill. And in dealing with South Africa, as one Republican put it, Reagan simply recognized "the reality of having a political gun at his head."

Striking a bargain at Geneva involving SDI would appear to be far more difficult than Reagan's past bargaining successes. Soviet suspicions about Reagan and the president's distrust of the Soviets run deep. The issues involved are highly technical and thoroughly unresolved. So far, U.S. and Soviet negotiators in the strategic arms talks at Geneva have made no progress in narrowing differences on SDI.

"The president has made very clear that he thinks we not only have no alternative but to investigate the scientific feasibility of defensive systems but that it would be irresponsible not to do so," a senior administration official said Friday. "I'm absolutely sure that any agreement that constrains our ability to do that is unacceptable and should be unacceptable."

But this official added that the United States "would like to" discuss "the whole relationship" between offensive and defensive systems.

"The president is not only willing to discuss them, he wants to discuss them," the official added.

Some officials suggest that the very vagueness of SDI makes it ideal negotiating fodder for Reagan at Geneva. While insisting that his program proceed, Reagan has never said how much of it has to go on, or at what pace.

The Soviets also been vague. Gorbachev, in his interview with Time magazine last month, accepted the idea of "fundamental research," but did not define this term. Both sides have acknowledged that pure research cannot be monitored or effectively limited by international negotiations.

Reagan has lately used the phrase "research and testing," and McFarlane has argued that "testing" is permitted under the 1972 Antiballistic Missile Treaty that explictly bans "development" and "deployment." But neither the president nor his national security adviser have said how much testing is necessary or how long it should continue.

"Definitions are critical," an official said last week. "If we can come to some broad agreement on terms and if the Soviets are convinced that Reagan is both serious and flexible, then it might be possible for the negotiators at Geneva to work out a specific agreement after the summit. That would be a real breakthrough."

One problem with achieving such a "breakthrough," in the view of some participants in the process, is that Reagan may have oversold himself on SDI. One Republican congressman who thinks that SDI might be useful in protecting U.S. missile installations and in discouraging a Soviet first strike thinks that Reagan mistakenly believes, as he said at his last news conference, that it is the path to "entirely" eliminate nuclear weapons.

Privately, Reagan reinforces the view that he is heavily sold on SDI by referring to it as "a nuclear shield" or "umbrella" that will protect people as well as missile sites. Such statements, if repeated at Geneva, could reinforce the deepest Soviet fears about SDI and make practical negotiations difficult.

Nonetheless, officials say that Reagan has held discussions with his staff about limiting SDI, although not in great detail. One official who is aware of these discussions said that Reagan does not even want to acknowledge them because they obviously devalue a potential deal. This official said he believes that Reagan wants to be an effective negotiator with Gorbachev "and that means you don't show your hand early."

Some of those who think a swap of SDI development for cuts in Soviet offensive weapons is a realistic possibility are relying on McFarlane, who is believed to understand the potential of such a bargain, and on former president Richard M. Nixon, one of the outside experts whom officials say Reagan is consulting before he goes to Geneva.

In the current issue of Foreign Affairs, Nixon makes the case that "deployment of SDI , as distinguished from research, for defense of our missile fields is the ultimate bargaining chip . . . . We should agree to limit our deployment of defensive weapons only if the Soviets significantly reduce and limit their offensive weapons."

In addition to McFarlane and Nixon, Nancy Reagan and the Reagans' friend Deaver, who has been brought into the White House to help advise on aspects of "public diplomacy" before Geneva, are seen as favoring a bargain with the Soviets, if one is realistically possible.

"For a long time Nancy has seen the potential of Reagan being regarded as a 'peace president,' which she truly believes he is," said another friend of the Reagans. "She is very influential."

Whether anyone will be influential enough with Reagan to lead him to bargain away the active development of a system he has come to regard as the "great protector of our people" remains a large question. But despite Reagan's public statements to the contrary, there are those in his official family who think he is moving in this direction.

Staff researcher James Schwartz contributed to this report.