Last fall, when President Reagan's advisers wanted him to demonstrate a campaign commitment to peace by meeting with Soviet Foreign Minister Andrei Gromyko, they turned to Nancy Reagan for help -- and got it.

"Her involvement . . . was for the purpose of being reelected," recalls longtime Reagan strategist Stuart K. Spencer. "I took the position that it was political suicide to keep up the hardball rhetoric. We didn't talk about the merit of it, we talked about the politics of it."

The account of how Nancy Reagan sided with pragmatists against administration hard-liners to maneuver Reagan into a meeting with Soviet Foreign Minister Andrei Gromyko -- which became a public relations triumph for the president -- was told anonymously during the campaign. Tonight it is recounted with blunt and convincing force on camera in an engrossing NBC special, "The First Lady: Nancy Reagan" (Channel 4 at 10 p.m.).

What emerges from the skillful interviewing of NBC White House correspondent Chris Wallace is the portrait of a charming and calculating woman who behind the fac,ade of "adoring wife" has become a consummate politician and indispensable ingredient to her husband's success.

Former political adviser Lyn Nofziger offers the view that without his wife's help Reagan would never have become president. Present adviser Edward J. Rollins says, "I think that she wanted him to be president more than he wanted to be president." And consultant Spencer, testifying to the importance of the first lady's influence with those who remain in the president's favor, says that if she called and said, "Stu, I want you to jump," his response would be "How high?"

For once, the president's men are upstaged by Nancy Reagan, who drops her long-held pretense of being simply an old-fashioned wife and acknowledges a leading role in determining the cast that surrounds Reagan in the White House. Mrs. Reagan, who Rollins says has the influence to make any issue the focus of the president's agenda, clearly believes that her husband is too trusting of other people.

"I think I'm aware of people who are trying to take advantage of my husband, who are trying to end-run him, lots of times who are trying to use him," she tells Wallace. "I'm very aware of that. All of my little antennas go up."

Nancy Reagan is also aware of her own changing image, which Wallace traces from the "Queen Nancy" days of White House redecorating and china purchases to her present role as behind-the-scenes politician and public battler against drug abuse.

Her wifely image has changed, too. Instead of reverentially gazing at Reagan, she kids with him and even jokes occasionally at her own expense. In one memorable bit of byplay with Wallace, she coyly acknowledges that her official biography shaves two years off her real age (she is 64) and says "I haven't made up mind yet" which birthdate she will use.

Nancy Reagan also talks candidly about her unhappy childhood and, with genuine emotion, about her admiration for the late Loyal Davis, the Chicago surgeon who adopted her and became the shaping force in her life. Three of the Reagan children (Patti Davis refused to be interviewed) also demonstrate what might be called restrained candor in discussing their mother or stepmother. "She can be a handful," says Ron Reagan. "I mean, she's not always the easiest person to get along with . . . I don't think I'd want her to be my boss."

The program is not without blemishes. In the concluding and generally less interesting minutes of the special, Wallace presents some disembodied poll data that tell us less than he thinks they do about Nancy Reagan's popularity. Wallace also skirts a definitive answer to one of the most important questions he raises -- whether the change in the Nancy Reagan image is genuine or the product of pure political calculation.

The two sides of this equation are represented by Washington Post reporter Donnie Radcliffe, who says the only change is Nancy Reagan's realization of "how she comes across to the public," and Mrs. Reagan's friend and former White House aide Michael K. Deaver, who maintains that the first lady now realizes for the first time "that she can do something with her life independently which can make a change for the good."

Wallace's own view is that Nancy Reagan has her eye on history and that in the remaining years of the administration is likely to play "a bigger and bigger role -- committed not only to securing the president's place in history but also her own."

To those who have known Nancy Reagan since the days when she was a scared and largely apolitical wife of a novice governor of California, these views are not mutually exclusive. It is difficult to think of any political figure, except possibly Ronald Reagan himself, who came into government knowing so little and has learned so much.

By any measure, Nancy Reagan has traveled a very long distance. Thanks to Wallace, many of the milestones of this journey are compellingly presented on NBC tonight.