"Reagan chooses nitwit as minister," read the headline in an Amsterdam newspaper; and the London Daily Mirror commented: "America's allies in Europe will hope he is never in charge at a time of crisis."

These were only two of the brutally harsh and sarcastic judgments rendered by the world press on William P. Clark last February after he testified before the Senate Foreign Relations Committee on his qualifications to serve as deputy secretary of state. It wasn't one of Clark's finer hours.

He was unable to identify the prime ministers of South Africa or Zimbabwe and reacted as if he was hearing for the first time about such matters of international interest as the controversy over placing tactical nuclear weapons in Europe or the split in the British Labor Party. In fact, he seemed like one of the most spectacularly ill-suited candidates for a major foreign policy post in modern memory.

Even the normally mild-mannered committee chairman, Charles H. Percy (R-Ill.), who reluctantly voted to confirm Clark out a sense of party loyalty, later remarked acidly, "Never again can we accept a man who professes to have no knowledge in the area for which he has been nominated."

But that was eight months ago. In the time since, there probably has been no other senior Reagan administration official about whom opinions have changed more radically than Clark, a 50-year-old former California Supreme Court justice who is known throughout the State Department as "the judge."

Clark's name doesn't appear in the news very often these days. However, among people familiar with the inner workings of the administration's foreign policy machinery, he is widely regarded as perhaps the most influential and powerful man to occupy the State Department's second-ranking job since George Ball in the 1960s.

Clark's standing within the administration is so high that he frequently is mentioned as a potential successor to his boss, Secretary of State Alexander M. Haig Jr., or as a possible candidate for an important insider's slot at the White House. More recently, as the impression has spread that President Reagan's national security affairs adviser, Richard V. Allen, is on his way out, Clark's name keeps cropping up prominently in the speculation about likely replacements.

He got there partly by making up for his lack of expertise through sheer hard work and a punishing, on-the-job learning process. Although there are still a lot of blank spots in his knowledge of world affairs, Clark has parlayed his ability to learn by doing into a major role in the internal management of the department. He also has put his fingerprints indelibly on such controversial and high-priority administration policies as its approach to combating Cuban influence in the Caribbean and resolving racial tensions in southern Africa.

However, Clark's real importance rests in his ability to perform another--and unique--function for the administration. He is the principal buffer, interpreter, guidance counselor and damage-control intermediary between the mercurial Haig and the palace guard surrounding Reagan at the White House.

Clark has been able to fit into that role because he has won Haig's trust and respect, while retaining his credentials as a member in high standing of the tight circle of Californians--among them presidential counselor Edwin Meese III, White House deputy chief of staff Michael K. Deaver and Defense Secretary Caspar W. Weinberger--who have been among the president's closest political intimates since his days as governor of California.

It was Clark who moved in as Reagan's chief of staff in 1967 when the governor's office in Sacramento was in a shambles and made it run smoothly, who enlisted Meese and Deaver for the Reagan team and who spent his 1969 vacation in Washington helping Weinberger, then a Nixon administration official, reorganize the Federal Trade Commission.

As one long-time observer of this old-boy network points out, "What Clark lacks in knowledge of foreign affairs is more than compensated for by the fact that he knows every contour of the hearts and minds of Meese and Deaver and Weinberger and Reagan himself. He shares their conservative political outlook and their penchant for doing things in orderly, teamwork fashion. He speaks their language in a way that a temperamental type like Haig would never be able to do."

This ability to "speak their language" keeps Clark almost constantly on the telephone to his old cronies at the White House and the Pentagon, running interference for Haig on policy and jurisdictional disputes and smoothing over the dustups that sometimes have made Haig appear to be in open warfare with the rest of the administration.

Authoritative State Department sources say that at least twice Clark was instrumental in diverting Haig from a collision course that could have had disastrous results for the administration's image. Last spring, when Haig exploded in public anger at the White House's decision to give control of the government's crisis management machinery to Vice President Bush rather than to Haig, Clark sat with the secretary for hours and patiently talked him out of resigning.

Then, these sources add, during the summer, Clark stepped in again and gently dissuaded Haig from demanding that Reagan apologize personally for critical comments about the secretary leaked by White House aides to the press.

Clark's skill at this kind of conciliation helps to explain why most State Department insiders discount the rumors that he might move into the national security adviser's job if the president decides that Allen's problems over his dealings with former Japanese business associates make it advisable to drop him.

In part, many department officials note privately, Clark's education in foreign policy matters still hasn't gone far enough to really qualify him for a job whose main function is to keep the president informed and the rest of the administration coordinated on the broad range of national security business.

Only last week, during a visit to Ireland, Clark demonstrated that there still are some very rough edges on his grasp of sensitive issues. During an interview in Dublin, he used some carelessly loose language that implied the United States was departing from its policy of noninterference in the Irish reunification dispute and that forced the State Department to rush out a clarifying statement that U.S. policy remained unchanged.

But, these officials continue, the main reason Clark is considered unlikely to move to the National Security Council, if and when Allen leaves, is that the president and the senior White House staff consider him more valuable in his present position as their interlocutor with Haig.

It's a role that Clark didn't expect to be playing a year ago when his old friend, Reagan, won the presidency. He had left active politics in 1970 for a series of judgeships that culminated in his supreme court appointment, and he was content to divide his time between his cattle and grain ranch in California's San Luis Obispo County and his duties on the court, where he was known as a strict law-and-order conservative at frequent odds with the court's liberal majority.

His name immediately came up on Reagan's short list of candidates for such jobs as attorney general, secretary of agriculture and director of the Central Intelligence Agency, but Clark sent word that he was happy where he was and didn't want to be considered for any of them.

It wasn't until Meese made a cross-country trip to Clark's judicial chambers in San Francisco and advised him that the president wanted him to become deputy secretary of state that Clark reluctantly relented, even though, as he candidly admits, "I didn't know any more about the subject than any casual reader of Time or Newsweek."

Initially, everyone assumed that Reagan wanted Clark at State to act as a watchdog on the unpredictable Haig. But, after the fiasco of Clark's confirmation hearing, it seemed that the pugnacious, internationally famous former general would swallow the softspoken, judiciously quiet outsider from California in a single gulp.

When a reporter asked Haig at a cocktail party how he felt about having a deputy whose credentials seemed so threadbare in comparison to his own far-ranging experience in foreign policy and national security, the secretary grinned broadly and replied: "Actually, I feel pretty good about it."

In reality, State Department sources agree, the two hit it off well from the outset. Both basically shared Reagan's mistrust of communism and his desire to restore American preeminence in world affairs. Haig, instead of trying to shut Clark out, earned his gratitude and affection by taking the newcomer into his confidence and assigning him increasingly important responsibilities.

Clark himself recalls, "From the outset, the problems began piling up to the point where there was far more on Al's plate than he could handle. When something new came in that needed immediate attention, I'd say, 'Al, do you want me to take that one?' and he invariably would answer, 'Bill, could you do that for me?' "

Clark also insists that there never has been a conflict between his loyalties to Reagan and Haig. He says, "Since I've been here, Al has known of every communication I've had with the White House, both before and after; and he, in turn, has shared all his information and decision-making problems with me. We don't always agree on how to do some things, but we work together; and he's never held me away from a problem out of concern that I won't do it the way he wants."

Clark admits, "I had to start pretty much from ground zero and educate myself on subjects I'd never thought about before." But department officials at all levels give him credit for being a quick learner and, even more importantly, a man who could slice through bureacratic red tape and make his decisions stick.

In addition to his primary role of liaison with the White House and other government agency heads, Clark by now has staked out a number of little-noticed but very important functions within the department--ambassadorial appointments, internal management, the controversial shifting of human rights policy away from the activism of the Carter administration--as provinces where his is pretty much the last word.

Where policy initiatives are concerned, Clark has had a big role in two areas where the administration has generated a lot of controversy: Latin America and southern Africa. He has been the main overseer of State's efforts to launch the still evolving Caribbean basin initiative, aimed at combating Cuban influence in that region through development and trade assistance. A few months ago, he went to Pretoria for talks with South African Prime Minister Pieter W. Botha--"the man whose name I didn't know at my confirmation hearing"--to help break the logjam on negotiating an independence settlement for Namibia.

"That doesn't mean that the Caribbean or Africa are areas that have been assigned to me for my exclusive concentration," Clark says. "I have no priorities, and I try to be a utility infielder. More and more, my role inside the department is doublechecking and backstopping things that we're falling behind on in all areas."

"My job is trying to avoid what Al calls the grave error of the Vietnam era when the government got so focused on one issue that we lost sight of other things that were of concern to both our friends and foes. I got into the Caribbean basin thing because we wanted it on the president's agenda at a time when other people wanted to defer its consideration, and I went to South Africa because there was a danger that the Namibia negotiations would get frozen by inertia to the point where the opportunity would be lost."

"Now," he adds, "these matters are on track, for the moment at least; and I can get into other areas that need attention."

As to the future, Clark professes to miss life on his ranch, which is being run for the time being by the eldest of his five children, and talks about getting back there "before too much longer."

However, others see different scenarios for Clark's future. Within the department, there are many career officials, impressed by his access to the White House, who would like to see him become secretary if and when Haig calls it a day. Others, including some Republican politicians, note that Meese reportedly would like to end his service in the Reagan administration as attorney general and say, if that happens, Clark could wind up as Reagan's right hand at the White House.

For the present, such ideas remain in the realm of sheer speculation. What is clear, though, is that both Reagan and Haig seem very happy having Bill Clark sitting in his seventh-floor State Department office ready to pick up the telephone when some smoothing out is needed, and both probably would go to very great lengths to keep him there.