Soviet leader Mikhail Gorbachev's secret weapon in the Star Wars opening here Tuesday may be his wife Raisa, but there is nothing secret about Ronald Reagan's. She is Nancy Reagan, now regarded inside the White House as a proven commodity in her ability to grab favorable headlines when the going gets rough for her husband.

As the other half of the team presidential aides refer to as "Reagan and Reagan," Mrs. Reagan has a strategic role to play -- both on and off prime-time television -- at the Reagan-Gorbachev summit.

Americans watching TV won't see her in the private role, the one Reagan aides say she plays best: that of presidential confidant and sounding board, someone to whom Reagan will turn with observations about his meetings with Gorbachev or with questions about the character and motives of the people involved.

What they will see is Mrs. Reagan on an international stage in a carefully thought out public role, one designed to provide what one insider calls the "warm, fuzzy" side of the administration's image at the summit. She'll be cast as a hospitable humanitarian/diplomat to enhance Ronald Reagan's image as tough-minded statesman/negotiator.

Since the Reagans arrived here Saturday -- she wearing boots and a purple leather coat with fur collar -- they have been side by side. Today she accompanied him to help look over the Villa Fleur d'Eau room in which the talks will take place, sitting in Gorbachev's chair while Reagan tried his chair -- and prompting a presidential quip: "Well, you're prettier than I expected."

But while Reagan and Gorbachev talk Tuesday and Wednesday, Mrs. Reagan will go her own way in what aides expect to be a blaze of media coverage.

White House chief of staff Donald Regan said he expects the coverage of her and Mrs. Gorbachev's activities to have high appeal, especially to women.

"They're not . . . going to understand throw-weights or what is happening in Afghanistan or what is happening in human rights," he said. "Some women will, but most women -- believe me, your readers for the most part if you took a poll -- would rather read the human-interest stuff of what happened."

The high point of that coverage will come when she and Mrs. Gorbachev sit down Tuesday to have tea. The tea, which Mrs. Reagan initiated, will be the first meeting between Soviet and U.S. first ladies since Jacqueline Kennedy and Nina Petrovna Khrushchev met in 1961 at the Vienna summit.

"Since so much of summitry has to do with atmospherics and appearances and really less with the substance, a great deal of attention will fall on Mrs. Reagan and her encounter with Raisa," said Richard Allen, former national security adviser to President Reagan, in a recent interview.

It is an aspect of the Geneva talks that the White House has dared not overlook. Little available about Raisa Gorbachev, her interests and accomplishments, or her clothes, jewelry and behavior, has escaped Mrs. Reagan or her aides.

Mrs. Gorbachev, 52, is still an unknown quantity in the West despite reports of her savvy in political science and world affairs -- she is a PhD professor at Moscow University. Mrs. Reagan, 64, after four years of "on-the-job" training, is a seasoned "diplomat" with an ability to warm hearts and stir concerns over her highly publicized interests in family, home and drug abuse.

The summit assignment may be the most challenging yet in a year in which Mrs. Reagan has increasingly volunteered her services as a diplomatic go-between, sent off on her own as a player in the high-stakes game of international politics. Last month she was at the United Nations, hosting the first ladies of 30 countries at a conference on fighting drug abuse, a follow-up to a similar White House session with 17 first ladies in April. In September she flew to earthquake-stricken Mexico City, a condolence call White House aides judged a "major plus" for Mexican-American relations. In May, while Reagan was at the Economic Summit in Bonn, she traveled alone to Rome to meet privately with Pope John Paul II to talk about worldwide drug abuse.

This week, fully aware of the interest she and Mrs. Gorbachev will be generating together and apart, Mrs. Reagan will follow a well-established pattern of independent activities: something that involves children, a sampling of culture and an expression of interest in community concerns, usually drug abuse. This time it's a boat ride on Lake Geneva with American schoolchildren, a tour of a Swiss village and an alpenhorn band concert, a visit to a drug treatment center in Lausanne. She'll also unveil a sculpture in her honor and tape television and radio messages for UNICEF's worldwide vaccination campaign.

She will be seen with Mrs. Gorbachev at Tuesday's tea, Mrs. Gorbachev's reciprocal tea Wednesday, a Red Cross ground-breaking ceremony Wednesday, and -- with their husbands and others -- both nights at reciprocal dinners.

James Rosebush, Mrs. Reagan's chief of staff, said in a presummit interview that it was inevitable that the two women would be compared, but that he thought it should be done "carefully." Of particular interest to the White House will be Mrs. Gorbachev's Geneva schedule.

Today White House aides and the press were still trying to find out how she will spend her time while Gorbachev and Reagan are behind closed doors. In Paris and London she made headlines on what appeared to be media-targeted outings designed to show her as an attractive woman interested in clothes and culture. The generally favorable coverage that resulted raised a few eyebrows among Reagan supporters.

"It is interesting to me that here Nancy four years ago was criticized for clothes, and this woman was going to couture houses in Paris and buying Tiffany diamonds and so forth," said Michael Deaver, former White House deputy chief of staff and longtime Reagan aide. "It's incredible."

But the frivolous "Fancy Nancy" image of 1981 has been replaced with an image of Mrs. Reagan as a serious woman, concerned with humanitarian issues and interested in domestic and world affairs. This Mrs. Reagan does her homework. In preparation for the summit she got her own weekly briefing papers from the National Security Council and read several books about Russia, aides said. Rosebush headed Mrs. Reagan's advance team to Geneva, telephoning her daily and bringing back pictures so she would be as familiar with the surroundings as possible.

"We'll be pursuing the same course we have on every other foreign trip," said Rosebush, who admitted a certain fascination with Mrs. Gorbachev's visits to the St. Laurent and Cardin fashion houses in Paris. "I think Nancy Reagan's record will continue to be on the level of what she's done before -- more serious."

There is no agenda of suggested conversation topics when the two women meet for tea where the Reagans are staying, Maison de Saussure, an 18th-century residence rented by Prince Karim Aga Kahn.

Regan, who reportedly preferred a tightly focused summit with no wives present, said in a recent interview that he saw Mrs. Reagan and Mrs. Gorbachev here "to build bridges, talking as the wives of the two leaders as to how things could be done mutually."

He said he did not expect them to discuss the summit's substantive issues such as disarmament, international conflicts or human rights violations, but rather shared interests and concerns, perhaps "drugs and other common problems that affect people of both countries." Mrs. Reagan, he said, "doesn't get into throw-weights or warheads or methods of transporting these warheads."

But that doesn't mean Mrs. Reagan's teas with Mrs. Gorbachev or her other activities aren't part of the summit plan, he said.

"No, we think they are a very important part of it," Regan said. "This is not just a sideshow, far from it."

But there was at least some presummit concern that a sideshow could overshadow the main event -- that the wives' activities, or other public activities, might obscure the issues.

"I have no problem with Mrs. Reagan going to the summit, but I don't want it to be a diversion from what's really going on there between the two main players," said Rep. Pat Schroeder (D-Colo.), a member of World Women Parliamentarians for Peace.

"A danger associated with summitry -- therefore as much a danger associated with a first lady as the president -- is that you seek to run the summit, to direct its results, according to press flackery or public relations," Allen said. "I do not see that as a likely danger on the part of the president but I certainly do in other respects.

"I see it coming from other quarters -- those who are more interested in the domestic, political implication of this summit than in the substantive contributions to our national security and well-being."

Mrs. Reagan's activities, Rosebush said, are the kind that create a climate in which progress can be made on those substantive issues.

"Certainly the president has his very important role at the negotiations," he said. "But it should not be overlooked that things also can be accomplished between people, impressions conveyed and friendships forged and solidified in a more casual setting."

Rosebush said he doubted there would be any lulls in teatime conversation. He also predicted that the two women would not talk about clothes ("I've never heard Mrs. Reagan talk about clothes to wives of foreign leaders") or Marxism and Leninism, Mrs. Gorbachev's academic specialty.

"I think it should be open and freewheeling. My opinion is there will be no problem at all. What I've read about Mrs. Gorbachev and know of Mrs. Reagan, they're both interesting, full-of-life people who are going to have something to say to each other," said Rosebush.

Another place White House aides believe Mrs. Reagan will hold her own is at the dinner she and the president will give for the Gorbachevs on Wednesday, as well as the one the Gorbachevs give for the Reagans Tuesday. Neither dinner will be formal, and, at the Reagans' at least, a certain intimacy is expected since space is limited.

Falling to Mrs. Reagan, he said, will be the responsibility of creating "the atmosphere and the feeling in which a good result can occur."

Behind the scenes, her job will be similarly strategic.

In Regan's view, her biggest contribution at the summit will be "to reinforce some of the president's positions that he's going to be taking, encouraging him to be up for it, watching his moods, his health, his diet, his hours of sleep -- things of that nature so that he is in good shape.

"Remember," Regan continued, "these meetings are grueling on the nerves, physical wear and tear on the system. You have to be up for it. That's where she's superlative."

Because she is eager that her husband be known as "a man of peace," Regan said he expects Mrs. Reagan to reinforce the president's "natural inclinations in that direction . . . without giving away the shop. She'll definitely reinforce the advice that the president is getting from his staff."

But where she might use her influence, said Deaver, is to "soften the rough edges" of an adviser by telling her husband, "I don't think you ought to say it that way."

Deaver, who has known Mrs. Reagan for more than 20 years, said he doubts public opinion could sway her, though she might help her husband see a way to sway public opinion. He said she knows when to hold off and when to push, and aides who might try to use her as a means of promoting a certain position this week could find it "counterproductive."

Deaver echoes what Ronald Reagan himself often says: that wherever he goes he wants Nancy Reagan with him.

"From the standpoint of companionship," said Deaver, "and secondly as a sounding board, he recognizes her antennae and judgment and knows she's the one person he can trust."