THE SHOWDOWN came in Ronald Reagan's living room one balmy California Monday, an afternoon in 1979 right around Thanksgiving. Reagan was two weeks into his official presidential campaign, enjoying a front-runner's calm. His staff was at war.

In one overstuffed chair was campaign director John Sears, the strategist whom critics saw as Reagan's political Svengali. In the other was Mike Deaver, trusted aide and close friend -- the assistant who knew, if anyone did, the candidate's heart. Reagan had listened when Deaver suggested that Sears be brought into the campaign.

But that afternoon Reagan and his wife served as mediators. Sears, flanked by aides Jim Lake and Charles Black, complained that Deaver wasn't handling the campaign's money well. Deaver maintained this was just an excuse; the real problem, he said, was that the imperial Sears wanted sole power.

The talk twisted for four hours. And then, from the recollections of both Deaver and Lake, this is what happened:

"Well," one of the group said, "all we're doing is repeating ourselves." Drained and grim, they agreed it had come to either Deaver or Sears.

"Yes, honey," Nancy Reagan said to her husband. "You're going to have to make a choice."

Deaver stood up. "No, Governor," he said, "you don't have to make a choice. I'll resign." It was said in sadness, not anger. He walked to the front door. Reagan followed.

"I didn't want it this way," he told Deaver. His voice was agitated.

"This is the best way," Deaver answered, softly. He started to leave, but then remembered his wife had dropped him off. He needed a ride. Reagan loaned him his car.

Sears, Lake and Black were still in the living room when Reagan came back. Lake recalls the candidate's reaction: "Goddamn you guys," Reagan said in anger, "the biggest man in this room just left." The Body Politic

After that living room scene at the house in Pacific Palisades, Deaver left Ronald Reagan for what he said was forever. It wound up as six months. Sears was fired after the New Hampshire primary; Deaver came back at Reagan's insistence.

Standing outside the Oval Office on a recent afternoon, the president says simply: "It never should have happened."

Mike Deaver now works in the White House office that was once Jimmy Carter's study. Reagan gave him this, and the job of deputy chief of staff as well. It's a deceptive title; Deaver, along with Ed Meese and James Baker, is part of the triumvirate that White House staffers call the "Big Three." It's a loose arrangement, but generally, Meese handles policy, Baker is in charge of administration, and Deaver, the man himself. They called him "the keeper of the body" inside the campaign.

His duties are both nebulous and vital. Through a decade and a half, he's developed so much of a sixth sense about the president's stamina and work habits that Nancy Reagan calls him almost every day. Deaver advises Reagan on scheduling and television appearances; he also advises staffers on the president's prevailing mood.

"If I ever had any bad news to give the president," says White House speech-writer Ken Khachigian, "I'd like Mike to deliver it for me."

Deaver is a 42-year-old Californian with a sympathetic touch and irreverent wit. Last fall, he had a food fight with fellow staffers at a Middleburg restaurant. The last 10 years, he's listened to their sorrows."You could talk to him about problems and shed a few tears," says Elaine Crispen, the former secretary who did.

"I think I'm a sucker for a sad story," sighs Deaver. Once he studied to be an Episcopal priest. Still, he'd be hard to find in a crowd. If anything, he looks as clean-cut and efficient as an IBM management trainee, which is what he once was.

Distinguishing marks: crisp button-down shirts, slightly bald head, hunts ducks, plays piano. Member, Sacramento society. Green thumb, too. Last month, he fretted that the White House rosebushes weren't properly pruned.

But he's tough. He moved Lyn Nofziger out of campaign finance operations, and Nofziger got so mad he quit. White House confidants say he can tell you exactly who went for Ronald Reagan -- and when.

Now he is the closest personal adviser the president has. "I think Ronald Reagan looks at Mike Deaver as a son," says Nancy Reynolds, a friend who began working for Reagan the same year Deaver did. "I think Mike has telepathy with Ronald Reagan. He understands what he's going to say, think and do, sometimes before the man himself does."

"Gee," jokes the president, "I always thought of him more as a father figure." Deaver chuckles.

It is a warm, breezy day, and the two are standing on the terraced walkway just outside the Oval Office. It teases of spring. The men talk in soft, clipped sentences, a verbal shorthand that has evolved over 16 years. One begins a thought, and the other finishes.

Still, the relationship is different from what it once was. "I think it probably did change," says Deaver's wife, Carolyn.

A close White House friend puts it another way. "I never sensed any bitterness," says Caraig Fuller, the Cabinet secretary, "but something important happened to Mike as a result of that meeting. John Sears had questioned Mike's performance in the campaign, and caused the Reagans to doubt his commitment.

"So I think," continues Fuller, "that he learned what his relationship with the Reagans was really all about: Despite all his years of complete loyalty to the Reagans, someone could still create a doubt in the Reagans' minds about Mike's loyalty to them." Pomp and Circumstances

Lunch at Vincenzo, a Dupont Circle restaurant. Deaver is running late. "Margaret Thatcher kept on talking," he apologizes, refering to a meeting he's just left between Reagan and the British prime minister. Deaver handled scheduling for her visit, and is now well into plans for the president's Canadian trip.

He asks for a glass of white wine. Then he puts on reading glasses, peruses the menu, orders stewed eggplant and broiled fish. His car and driver are parked just outside.

Despite the perquisites and pomp of this new life, Deaver says he'll leave in a year and a half. He's said that before. In a way, he's like the son who knows that he should leave home, but doesn't.

"I'm going to have to get out," he insists. "This might not have happened to me if I hadn't left the campaign for a period of time. It made me realize I could be something else besides an extension of Ronald Reagan. I enjoyed my family life for the first time in our marriage. I also enjoyed making money.

"I was at a point in my life," he continues, "when I really wanted more freedom, and I think the president was very upset about it. He called me that night, came to my office the next day, and it was later, when John Sears tried to make him make the same choice between him and Ed Meese [during the New Hampshire primary], that the president looked back and said, 'My God.'" That's when Sears was fired, although he and his aides say the division had been building long before that.

"I think Mike would be far better off today it he hadn't come to Washington," says Sears, whom Deaver hasn't seen since the afternoon in Reagan's living room. "There are probably some people who can continue to lead a derivative life, but they are few. I mean, he's always enjoyed the closeness and all that, but he also realizes the years are going by."

The stories of that closeness span years in the development of an aide who was a young man at the beginning, middle-aged at this end. Some snapshots:

Deaver, saving Reagan from choking to death on a campaign plane five years ago. "Ronald Reagan was sitting there, drinking Coke and eating peanuts," recalls Nancy Reynolds. "And all of a sudden, in a split second, we heard this choking. His face turned red, and as it got darker and darker, we all froze. But Mike jumped up, pushed Nancy aside, and grabbed the governor." He used the Heimlich maneuver, and the peanut popped out.

Deaver, scrutinizing and finalizing the guest list for Margaret Thatcher's state dinner. He and Nancy Reagan, who dotes on him, spent 40 minutes selecting the names, giving importance to loyalty. Insiders say he's a master at tempering the first lady.

"She needs him as a sounding board," says Khachigian, the speechwriter, "and as someone who can do things for her -- 'Can you see that Mike's [Reagan's son's] airplane gets here on time?' to the most sophisticated scheduling problem, to somebody has asked her to do an interview, and what does he think about it?"

"I've seen her flash of anger," says one staffer from the campaign, "and Mike has a way of diffusing it. He's a good buffer."

Deaver, commuting for several years between his family in Sacramento and his business in Los Angeles -- the Deaver & Hannaford public relations firm that had little more than Reagan as its client. He co-founded it with Pete Hannaford in 1974. Deaver never found a house the family wanted and could afford in L.A., so he took a small apartment and made the flight home on weekends.

"It gets wearing," says Hannaford of the one-hour commute. "After a while you either get a divorce or move." When Devers's commute stopped, the 1976 campaign began.

Deaver, reportedly yelling at Jim Baker's staff when they loaded Reagan up with briefing books -- and on the night before his economics speech. "I didn't yell at them," Deaver counters. "I just pointed out that you can't send up a major packet of stuff on the night before the address to Congress.

And Deaver, receiving a call from Reagan when he first heard that Carolyn, Deaver's wife was pregnant. It was 10 years ago. "I remember he called and said, 'What do you want? A girl or a boy?'" says Deaver. And I said 'Oh it doesn't matter.' And he said 'No, you should want a girl. because if you have a little girl, you get to watch your wife grow up all over again.'

"And," says Deaver, "he was right." So Long Sacramento

Deaver and his wife have two children: Amanda, 10 and Blair 5 1/2. They live in a house that could be called cute, but expensive, just off Foxhall Road. The living room is done simply in neutral colors, sparked with warm red furniture. There is plain white china in the kitchen, a basket of grapefruit and oranges on the table. It smells like fresh paint.

Carolyn Deaver could be called cute, too. She is petite, with short dark hair, warm eyes and a little-girl voice. She jogs, has run half-marathons, and is astounded by Washington. "It's totally different than anything else I've been used to," she says. "There is so much going on in the evenings."

For the deputy-chief of staff and Mrs. Deaver, certainly. One recent week they had four black-tie events scheduled. "This town," says Deaver amused. "That's all they do -- dress in black tie." Already, there have been three formal dinners at the White House.

"I think it's very flattering, really," says Carolyn Deaver. "There isn't time in the day to do it all."

But she had a hard time leaving their home in Sacramento. And she enjoyed those six months when her husband, in the words of Craig Fuller, was able to "wake up in the morning and not wonder where Ronald and Nancy Reagan were."

"One of the things Mike dislikes in the newspapers," continues Fuller, "is this 'Mike Deaver, Reagan aide.' His wife has chided him -- 'When will you ever stop being 'Reagan aide?'"

"I can't remember ever saying that," responds Carolyn. But she will say that those six months were "a very revealing period for both of us. Is was almost eerie, it was so quiet. All of the phone calls were cut down.But then, it was kind of nice . . . we had time to have longer talks. I think it really brought us closer together."

Carolyn and Mike Deaver met in 1967. She was a secretary in the governor's office and Deaver, her boss. One day she came in and asked for a raise.

"She was the cutest thing I'd ever seen," says Deaver. "And I said, 'I can't give you a raise, but I'll take you to dinner.' We were married two months later."

Both are from California. Deaver grew up in Bakersfield, the son of a Shell Oil Co. distributor. He went to San Jose State College, did time in the Air Force Reserve, then joined up with IBM for a year and a half. He got out, fast, when the boss called him in one day and told him to wear white shirts, not blue.

So, he withdrew $5,000 from his bank account and went around the world with a Delta Sigma Phi fraternity brother. They bought a Land Rover in London, them went on safari in Tanzania, took a boat from Naples to Alexandria. A fine time. Or at least it was until Australia. That's when the money ran out.

First, they shipped the Land Rover back to the fraternity brother's dad in the States. C.O.D. Deaver got a job playing piano, he says with a flourish, in the "Roseville Return Servicemen's Club."

"Wasn't a lot of money," he says, "but lots of good beer."

Eventually, he made enough to get home. He began working his way up through Republican politics, a behind-the-scenes star who was assigned five California House seats to win for the party. He got three.

Then the governor-elect's office called, and for eight years, he advised Reagan. He came to admire the man and his philosophy; that man, in turn, came to depend on Deaver's quick judgements and straight talk. A bond grew.

In 1974, the last month Reagan was in the governor's office Deaver began devoting most of his time to his new firm. Besides Reagan, his public relations business eventually has 20 other clients -- inlcuding Rockwell Interntional, the 3M Co., former agriculture secretary Earl Butz, former Minnesota senator Eugene McCarthy, and the government of Taiwan.

The last client caused a conflict-of-interest controversy. In recent years, at the same time Reagan was advocating the restoration of U.S. relations with Taiwan, Deaver and Hannaford was paid $5,000 per month by that govenment in fees. "There was nothing improper," Deaver said last June, "or we would never have taken the account." iBut because of possible future conflicts as a top White House aide, Deaver recently sold out his interest in the firm. Now it's called Hannaford Co. Inc.

In 1976 and 1980 he took leaves for the presidential elections. Except for that six-month gap in the more recent campaign, Deaver rarely left the candidate's side.

"Mike was the glue that held it all together," says Fuller, repeating a campaign cliche. But maybe the truth, too: Deaver was the first staffer to see Reagan in the morning, and the last to see him at night. Brief Encounters

He still is. His day begins at 7 a.m., the time his driver comes with the car. By 7:15 he's at the White House gate. At 7:30, he has a breakfast meeting with Meese and Baker, then a senior staff meeting at 8. At 8:45 the Big Three brief the president who's usually just come to work.

If it's to be a long briefing, they sit on the couches in the Oval Office.

If short, they stand three abreast facing the president's desk. On one recent morning, Meese was at left, Baker in the middle, Deaver on the right. rA gray morning light came if from the huge windows, touched slightly by rain.

The ceremony is almost overwhelming. But the four men communicate, as Deaver and Reagan sometimes do, in the short, bland sentences of business speak.

Meese: "Can we have that at, ah . . . how about 3?"

Deaver: "Have you got anything else on his schedule?"

Reagan: "Does this take any action?"

Reagan again: "Oh, I see you're sending me a letter . . . and the daily report. Oh, this is more of that correspondence." He shuffles some papers.

Meese: "Yeah, Max Friedersdorf's [the White House congressional liaison] following up on these items."

Meese leans one arm on Reagan's desk, Baker points to papers the president needs to see, and Deaver stands casually, hand in a pocket.

Deaver to Reagan: "You did a good job with Brinkley last night." He means an interview with David Brinkley, host of "NBC Magazine." More papers shuffle.

Meese again: "We had a breakfast meeting at 7:30 in the Cabinet room."

Reagan, sighing, shaking his head: "Seven-thirty . . ."

Meese: "Incidentally, we'd like to schedule Cabinet meetings in the afternoon."

Deaver: "Okay." He nods.

It is clearly Meese who is leading this briefing. Baker defers some, but Deaver defers more.

"Mike ought to have a better opinion of what he can contribute," Sears says, referring to the 1980 campaign. "He rarely will get involved in issues. sHe has a comfortable image of what he can do and can't do. Mike, if he errs at all, errs on the side of not thinking he can do as much as he can."

Deaver doesn't deny this. "I hang back more often than not," he says. "It's my nature. But maybe it's that while Ed and others concentrate on issues, I really instinctively try to see things through his eyes, through Ronald Reagan. How he would do it, what would be best for him."

Most of the rest of Deaver's day is taken up by more meetings. At 10 a.m., on this particular day, he goes to see Nancy Reagan at the residence. Lunch is at noon, Maison Blanche, with Judy Woodruff of NBC. Deaver says he likes to eat outside the gates at least twice a week.

At 3 p.m., he meets with the Secret Service. At 5 p.m., there's Pendleton James, the personnel director. At 5:30 p.m. he, Meese and Baker see the president again. At 6 p.m., the Big Three meet one last time. Deaver claims that despite the overlap of responsibility, the triumvirate is working well. He also quickly dismisses, as he strikes an easy-going air, suggestions of a future power struggle.

"Baker's a secure person," he says. "Meese is a secure person. They're not looking for some larger role." Asked to define his, he says casually: "I spend most of my days staying on top of the schedule, making sure it runs right. It took a couple of weeks to get the system going."

He loves the ambience of the White House. He has beautiful oil paintings on his office walls, a good-smelling wood fire when it's the least bit chilly, and a small patio just outside the paned glass doors. Peek over a hedge and you can see the Rose Garden. "And look at all these bulbs coming up," he says, poking around in the moist earth edging the patio's flagstones. "Everything. Tulips, daffodils, freesia."

"If Mike had his druthers," says his wife, "he'd be the gardener for some large estate."

During White House meetings, Deaver is known to insert sly witticisms into the stale air of conference. And the other day, when he was talking to a photographer about how a picture should be taken of him and the president in the Oval Office, Deaver suggested: "Well, I could stand outside and look through the window like this." He puts his thumbs in his ears and wiggles his fingers. Then he sticks out his tongue.

"He's irreverent," says one associate, "but that masks a deeper sense of respect. I think he's the one person who's still awed by all this, and I think he feels a sense of privilege to be here. There's no swagger."

In fact, at the party the Reagans gave their first day at the White House, Deaver was ecstatic. "Oh, my God," he laughed. "We're all about 10 feet off the ground." Tough Break

Which makes it that much harder for him to leave, as he insists he will. When he talks about it, and the good times of the past, he seems wistful.

"All those hundreds of hours, flying around on airplanes, just the two of us, or the three of us, with Nancy," he says. "And all those weeks over the last few years, when I've spent more time with the Reagans than I have my own family.

"Sure," he says softly. "It'll be hard. Absolutely."