He's one of the last of his class -- the patrician diplomat, tight-lipped and well bred. Harvard '28, a member of Porcellian Club, the elite men's club there. At 77, his hair is the texture of fine gray silk, and on his weathered, chiseled face is his history: Kennedy and the Cuban missile crisis, Eisenhower and Korea, meetings with Churchill and agreements with Brezhnev. More wars, more deals, more secrets. But it's the Soviets who are the toughest. Usually, when he is dealing with them, he has a piano sent up to his hotel suite and plays for relaxation. "The negotiations can be quite tense; they can put a lot of pressure on you, yes," he says.

For Paul Nitze, regarded as the most experienced practitioner of national security affairs alive today, there have been five decades of such negotiations, and Monday in Geneva could be the climax of his 45-year government career.

The veteran negotiator with a bipartisan re'sume' as long as the Kremlin Wall will be the special adviser to Secretary of State George Shultz when the United States and the Soviet Union dramatically discuss the possibility of reopening arms talks. And up until this week, Nitze was the favorite to become chief negotiator for possible subsequent talks, but has declined because of an illness in the family.

"That was never the deal," said Nitze. "I was asked to be an adviser and that's what I'll be."

At a time in history when the administration's commitment to arms control has never been as seriously questioned, the presence of Nitze, a registered Democrat who is regarded as someone who wants an agreement, has been seen as a signal that the administration is serious this time.

"Can the Russians be trusted? I'm not going to get into that kind of a question, about whether the Russians can be trusted," Paul Nitze is saying in his new office right down the hall from the secretary of state. "It's important to get to know these people, and get to know them in various different contexts, not just the context of the negotiations. It helps to understand . . . You learn something about a person by the character of his wit, for instance . . . And one also learns something from their wives and their children."

That's not to say Nitze is regarded as something other than a hard-liner by liberals who have never forgiven him for his strident opposition to the SALT II treaty, while the true hard-liners are still suspicious of him for his role in bringing about SALT I.

"I am comfortable with him considering what the choices could have been," says Sen. Gary Hart (D-Colo.), a member of the Senate Armed Services Committee who has had dealings with Nitze over the years. "I think he is positive, and flexible. He wants this to work."

"He's the best man available," says Eugene Rostow, former head of the Arms Control and Disarmament Agency and a longtime Nitze colleague. "That's all there is to it."

It is not an unusual job for him, having been through SALT I, START and most recently the INF talks on European missiles, which the Soviets walked out on a year ago. What is unusual is how it came about.

A few weeks ago, President Reagan formally appointed Nitze a "special adviser" to Shultz for the purpose of these talks. First of all, presidents don't normally formally appoint advisers for a single project. Second, the formal announcement, at the request of Shultz, was seen as a victory for the State Department in the battle between what has been portrayed as the moderates at State and the hawks at the Department of Defense who are known to favor rearmament, rather than disarmament.

It was also seen as a remarkable comeback for Nitze, who was thought to be in the doghouse with the more conservative factions of the administration because of his freewheeling negotiations with the Russians in the summer of 1982.

"There's really no substance to that at all," says Richard Perle, an assistant secretary of defense who is widely regarded as a hawk, and who is one of the principal combatants in the State-Defense war. "I really have a high regard for Paul."

It all started in Geneva, the summer of 1982, when Nitze headed the U.S. team for the INF talks to negotiate the future of the European missile arsenals for both sides.

After months of intense negotiations, Nitze's breakthrough came one July day, when he and his Soviet counterpart, Yuli Kvitsinsky, worked out an informal agreement during their "walk in the woods" after lunch at a mountainside restaurant. Hard-liners say Nitze went further then than his professional duties allowed.

"I did not," says Nitze. "And the president did not think so."

"He certainly didn't," says Rostow. "Neither one of us overstepped. Those who accused us of that, exceeding our instructions, failed entirely to understand our efforts. Every ambassador is supposed to poke around . . ."

"A lot of us were taken by surprise and dismayed by the procedures," says Perle today, referring to the "walk in the woods." "He felt that his instructions permitted him to proceed as he did."

The incident intensified the battle lines over foreign policy in the administration, with the more conservative factions succeeding in having Nitze's bargaining power limited.

Then-national security affairs adviser William Clark reportedly wrote a memo of complaint to Shultz against Rostow and Nitze. And a recently published book, "Deadly Gambits," by Strobe Talbott, maintains that Perle was also one of Nitze's biggest critics with regard to the "walk in the woods" and other aspects of the negotiations. Nitze, in turn, at one point accused Perle of trying to "torpedo" the negotiations, according to the book.

Nitze says all parts of the book that involve him are accurate. Perle says he is "unhappy" with how the book portrays his relationship with Nitze.

The Russians ultimately rejected the result of the "walk in the woods," and soon after, reacting to the U.S. deployment of European missiles, walked out of the talks. Nitze returned to the United States, and has been working at the Arms Control and Disarmament Agency. He was never formally reprimanded for his "walk in the woods," but nonetheless, some say he was humiliated by the mere fact that his judgment had been questioned.

Referring to Nitze and Rostow, Talbott writes: "They both regard themselves as senior statesmen in an administration overpopulated by pygmies in high places.

"That was his characterization," says Nitze, laughing. "Not mine."

From the start, Nitze has led a privileged life of success. After graduating from Harvard cum laude, he spent a financially fruitful decade as a Wall Street investor, amassing a fortune that has allowed him the luxury of a career in public service.

Many say he fits his diplomatic role: Reserved and handsome, he exudes a certain authority and presence. His hobbies are far from plebeian -- skiing in the Alps, sailing in Maine, horseback riding everywhere. He is married to the former Phyllis Pratt, whose father was a founder of the Standard Oil Co. of New York, and they have four grown children.

During a recent interview in his office, Nitze was clearly uncomfortable with any questions that required him to give an opinion on anything.

Are the Soviets different from others in negotiations? he is asked.

"Yes, they are, quite different. After one has worked with them for 40 years, one gets to understand them."

How different?

"That's a long story. I'm not going to get into it. Everyone knows the Soviets are the Soviets. They are different."

Can the Soviets be trusted?

"Oh, I guess they can. I don't want to get into that either."

Are they ready?

"They're ready, they're always ready," he says, "to support their viewpoint."

He first entered the government in 1940, as a Republican. "I was cross with President Eisenhower, so I switched," he says. "He failed to support Secretary of State George C. Marshall against a McCarthyite attack, and because he accused Democrats of tremendous political errors in withdrawing troops from Korea before the Korean War. I had a big fight with then-General Eisenhower when he was chief of staff in the Army. I thought we shouldn't withdraw troops from Korea and he insisted upon it. And then for him to say it was a big error on the part of the Democrats was absolutely unforgivable. So I switched."

Over the years he has been involved in the creation of the Marshall Plan, was present at a meeting with John F. Kennedy when the United States formed its response to the Cuban missile crisis, and was the principal negotiator on SALT I, after stints as secretary of the Navy and deputy secretary of defense in the Kennedy and Johnson administrations.

Two years after resigning as the Pentagon representative on SALT II because he thought the Watergate scandal might affect the talks, he formed the Committee on Present Danger and lobbied ardently against the final package, which he believed to be too soft. Jimmy Carter ultimately withdrew the second strategic arms limitation treaty from congressional consideration when the Soviets invaded Afghanistan, but Nitze's hard-line opposition to it has given disarmament advocates cause for concern.

Nitze doesn't regard this new assignment as any sort of a vindication, but rather the appointment of a pro.

"I've just been at it longer than most," he says. "I started negotiating with the Russians when I was in charge of a delegation in 1946. I was handling economic affairs in the State Department. In those days negotiations had to do with Article 12 of Lend-Lease, both sides sitting down together after the war to see if they could work out ways and means of making trade between the two countries. That's when I first negotiated with the Russians and I didn't get very far."

He says he brings to these negotiations most of all his expertise.

"Well, Shultz asked me to help him and I think I can," he says. "I'll do my best."

And does he also bring his much ballyhooed sense of caution, and skill in playing it close to the vest?

"Why, yes," he says, "I do tend to always win at poker and bridge."