First Lady Nancy Reagan's medical crisis comes at a time when President Reagan appears especially dependent upon her counsel and friendship.

The relationship of the Reagans, who have been married for 35 years, has long been considered one of the closest in politics. But friends of the Reagans say the 76-year-old president has become even more reliant than usual on his wife during the past 18 months because of the departure of key aides and the pressures of the Iran-contra investigations.

"She is the president's closest friend," said a Republican with ties to the White House. "He always relied on her, and with {former deputy chief of staff} Mike {Deaver} gone he relies on her even more. He tends to retreat into the residence or to Camp David and talk things over with Nancy if things aren't going right."

This source expressed the view that the First Lady's possible operation would not in the short term distract the president from decision-making in the Persian Gulf or elsewhere because "he is always such an optimist that everything will turn out well." Typically, the president yesterday expressed "great confidence" in the doctors attending his wife.

But friends acknowledge that the president could lose his focus if the operation and the First Lady's recovery do not proceed smoothly. "The president has a reputation for not worrying about anything, but he is concerned if Nancy even has a cold," said a friend.

While angrily denouncing stories that have depicted his wife as having undue influence, Reagan has never made any secret of his affection for her or of the value he places on their relationship.

"How do you describe coming into a warm room from out of the cold?" he asked in a 1968 interview. "Never waking up bored? The only thing wrong is, she's made a coward out of me. Whenever she's out of sight, I'm a worrier about her."

The Reagans are so close that she has on several occasions slipped him lines when she thought he was about to fumble a question. These assists have sometimes been interpreted as a sign that the First Lady is "running things" but historian Garry Wills sees them as reflecting a symbiotic relationship based on the mutual acting backgrounds of the Reagans.

"Helping another actor when something goes wrong, keeping the show going, covering an awkwardness is instinctive and shows not domination but the joint effort of the troupe," Wills wrote in "Reagan's America: Innocents at Home". "Reagan has done the same for her through the years, when a touch of her asperity at the press comes through, and he softens it with a joke."

As Wills sees it, the First Lady's principal role has been to bolster Reagan for presidential performances and to restore his morale when he is dejected.

But she also has been a key political adviser, particularly if she thinks aides are not looking out for the best interests of her husband.

"I think I'm aware of people who are trying to take advantage of my husband, who are trying to end-run him . . . who are trying to use him," she told NBC News White House correspondent Chris Wallace in 1985. "I'm very aware of that. All my little antennas go up."

In this same NBC News report, political consultant Stuart K. Spencer said of the president, "He's a patsy in personal relationships. He's easy. He's tough on ideological questions and tough on policy questions, but he has a very difficult time with personnel questions."

With this difficulty in mind, Nancy Reagan intervened during a crisis in the 1980 presidential campaign and helped oust campaign manager John P. Sears, who was replaced by William J. Casey. She subsequently played a role in several White House shakeups, most recently when Donald T. Regan was forced out as chief of staff early this year.

Soon after the secret U.S. effort to trade arms for hostages became public in last November, Nancy Reagan became convinced that Regan was mismanaging the White House response and had become a detriment. She brought in Deaver, Spencer, Democratic power broker Robert Strauss and others in an effort to convince her reluctant husband to fire Regan, who finally was replaced by Howard H. Baker.

The Regan ouster and subsequent criticisms of Nancy Reagan as a "power-hungry First Lady" -- a phrase used by New York Times columnist William Safire -- drew a heated response from the president, who said reports that she is "a kind of dragon lady" with policy influence were "despicable fiction." In fact, there are many close to the administration who credit the First Lady with encouraging the president's arms control efforts with the Soviet Union, which now appear for the first time to be bearing fruit.

Nancy Reagan herself joked about this on the day her husband nominated Robert H. Bork to the Supreme Court. Asked by a reporter if she had anything to do with it, she quipped, "No, I'm too busy with arms control."

There is widespread agreement in the White House that Nancy Reagan has been even more influential since Deaver, who has almost a familial relationship with the Reagans, departed his White House post in May 1986. Regan's ouster also boosted her influence, in part because the president does not have the joke-sharing, locker room relationship with Baker that he shared with Regan.

After the Tower commission's report appeared last February criticizing Reagan's "management style," the president vowed to become more involved in decision-making. Aides say he has genuinely tried to keep this promise. Few of them doubt, however, that he would be distracted beyond measure if anything happened to the person he values most.