No rabbi in the world plays such a pivotal role in his country's foreign policy as Moses Rosen, chief rabbi of Romania. And no Jew enjoys a comparable position anywhere in the communist world.

"He is Romania's most astute unofficial ambassador in the West," says Herbert Kaiser, former No. 2 at the U.S. embassy in Bucharest. "He is charming, knowledgeable, brilliant. He has done a lot to improve Romania's relations with the United States. And with Israel."

"He is a collaborator, a tool of the government, a dirty man," says a Romanian e'migre' who asked to be anonymous and who claims he speaks for "most Romanian Jews." "Whatever moves he makes, he has to ask for the government's permission. Otherwise he won't have his position."

"He is a courageous guy," says Philip Klutznick, an American Jewish leader whose last government position was that of secretary of commerce in the Carter administration. "He has done an enormous job in Romania, and I have a great deal of affection for him."

"We have to cooperate with the Romanian government," Rosen declared recently with an orator's solemnity. He was in Washington to attend a meeting of the World Jewish Congress, perhaps the most influential international Jewish organization. An earlier visit, last August, was to take part in what he calls "the most-favored-nation festival": He lobbied key congressmen, Jewish leaders and White House officials to make sure that Romania's controversial human-rights record would not bar its eligibility for those preferential credit and tariff terms known as MFN.

"I did more for MFN than the Romanian government asked of me, because I am convinced it's in the vital interest of the Jewish community," he said. "There is no choice other than cooperation. I am not a communist, and I have a philosophy of life that's opposed to communism. But if we want to remain Jews, we can't do anything against the government. Cooperation is to our advantage--and to their advantage."

Dressed in black--a cashmere overcoat, a chalk-striped suit and a homburg with a velvet skullcap underneath--he is a fashion plate of an Orthodox rabbi. At 71, he is pear-shaped, and has a firm, warm handshake and a ready smile. When he laughs, he thrusts his head backward and rolls his eyes. He speaks decisively in Romanian, Yiddish, Hebrew, French, German and English. He loves to talk, darting from one language to another; his repertoire of stories spans from the Bible to contemporary Eastern Europe.

In 1948, when Rosen became Romania's chief rabbi, he presided over 600 rabbis serving a community of 400,000.

Now, 35 years later, there are only three elderly rabbis and 30,000 Jews. They fall into two categories, Rosen said: those too old to start a new life in another country and those who plan to leave, once they see that the government is likely to approve their exit applications. According to Rosen, more than 60 percent of Romanian Jewry is over 60 years old.

Rosen is optimistic that a solution will be found in the most recent disagreement concerning MFN: The State Department warned recently that Romania's trade privileges might be canceled because of a new Romanian law that requires emigrants to repay the government for all state-provided education beyond the compulsory 10 years.

Reached in Israel, where he stopped on his way back to Romania, Rosen acknowledged that a serious problem had arisen in the past few days. But he cited assurances he had received last November "from the highest authorities" that the new law would not apply to Jews. "They agreed with me that aliya settling in Israel is not emigration," he said on the telephone. "They agreed with me that a Jew going to Israel after the Holocaust to rebuild his life is not emigrating . . . Jews are accustomed to being discriminated against. This was the first time that discrimination was in our favor . . . But now something changed. Now my information is that everybody has to pay the diploma tax. I am going back to Bucharest and I will do my best to contact the government."

What matters, Rosen declared on the telephone and again and again in the Washington interview, is that this year alone 700 of the country's 30,000 Jews have already received their passports and another 700 have applied for emigration. "Jewish emigration is continuing," he stressed on the phone. "The new law concerns general economic and political relations with the United States. The Romanians are humiliated and exasperated. Even a rabbi bringing the victory of MFN is a humiliation. It is a test. I am hopeful of a clarification."

"The application process takes time," he said here, shrugging. "It's up to the bureaucracy. And our people do not rush; they wait for their children to finish their studies. But nearly all of them go to Israel--it's because we educate them to be Jews. Unlike Soviet Jews who go to America."

The fact that he has presided over the disappearance of his flock is Rosen's greatest pride. As for himself, he and his wife plan to retire in Israel. (They have no children.) He owns an apartment in Tel Aviv, he said, and for years he has had an agreement with the Romanian government that he may leave whenever he wishes. The reason he hasn't left, he said, is that there is no one to take his place to head the community. "It's a one-man show," he said. "It's not good. And this one man is not a young man."

American readiness to cancel preferential tariffs and loans to countries with repressive emigration policies has given Rosen a position of strength in dealing with the communist regime. The United States is Romania's second most important trading partner in the West: In 1981, Romanian exports to the United States were close to $600 million. In 1982, a Romanian trade surplus of $130 million was critical to the country of 22 million people, which has at times failed to meet payments on its huge international debt of $13 billion.

In politics, Romanian leaders declare that good relations with the United States "balance" Romania's membership in the Warsaw Pact and form "the keystone of Romania's independent foreign policy." Israel maintains an embassy in Bucharest, and the two governments maintain strong economic and political links.

"Because the government gives us an opportunity to be Jews and the freedom to proclaim our love of Zion, I am a loyal citizen of Romania," Rosen said. "They let the American Joint help us." (The New York-based Joint Distribution Committee, a major American Jewish charity organization overseas, spent $4 million last year on programs in Romania.) "We have 11 kosher restaurants where our people eat fine lunches, better food than what the government-owned canteens have. We educate our youngsters in Jewishness--which is against the law. Who is doing such a thing in a communist country? We have thousands of tourists going to Israel--and thousands of tourists come from Israel. Alone in Eastern Europe, Romania has diplomatic relations with Israel!

"For all these favors we have to pay them. My duty is to pay them.

"Doing what I am doing is not only an act of good citizenship. Not doing it would make one a traitor to my community.

"I prefer an atheist regime doing the will of God by saving our lives to a priest who pretends he is a man of God but destroying us."

Rosen defends President Nicolae Ceausescu's "strong internal measures," which others condemn as Stalinist repression. " Soviet Communist Party Secretary Yuri Andropov wants to squeeze us," he said, and put his hands around his throat. "Ceausescu has to be strong. I respect him because he believes in what he is doing. He could live a comfortable life like Czechoslovak Communist Party Secretary Gustav Husak. But he works day and night. He attends meetings, talks to people, sends emissaries to make peace in the Middle East."

Rosen said that before the late Egyptian president Anwar Sadat made history by flying to Jerusalem, Ceausescu had spent six hours talking to him--ever so patiently--about the Israeli willingness to evacuate the Sinai in exchange for a peace treaty. And when the entire Soviet bloc broke off diplomatic ties with Israel after the 1967 war, only Ceausescu had the courage to dissent, Rosen said. According to Rosen, Ceausescu told the Soviet premier that if the Warsaw Pact governments could maintain normal relations with the United States despite the bombing of North Vietnam, a fellow socialist state, they should not punish Israel for fighting nonsocialist Arab states.

But what about those who disagree with the Romanian government, as Jews and/or Romanians? What about dissidents who see the chief rabbi's bargain as one with the devil?

"Let them be slaves," Rosen thundered. "It's not my business. If he is a hero he can fight the government.

"My life is dedicated to the Jewish people. Jews once fought for communism, but should they now fight against communism? I ask: This weak, small Jewish people should try to destroy the communist regime? Fight for peace and social justice in a time of jungle?"

Rosen deeply believes in the East European Jewish philosophy that Jews should not provoke the non-Jewish world. He calls the Jackson-Vanik amendment--which established a direct connection between emigration rights and most-favored-nation status--"not only immoral but stupid." Shaking his fists and wagging his finger, Rosen argues that under no condition should Jews lend themselves to be portrayed as potential scapegoats for economic problems.

Rosen beams with pride as he describes himself as the only chief rabbi in the Soviet camp who succeeded in convincing the Communist Party that it is no treason for a communist or a nationalist to have a Jewish education, to study Jewish culture and to desire to go to Israel.

"I want to be modest but not too modest," he said, grinning. "Little by little I was able to change the atmosphere in Romania. And things were terrible for Jews when I became a chief rabbi, in 1948, worse than in Russia. The improvement began in 1961, my first visit to America."

When in America, Rosen visits White House aides, high State Department officials and key senators. Earlier this month he was part of a five-man World Jewish Congress delegation meeting with President Reagan. On the substance of his contacts he is excruciatingly evasive. He is afraid that the press will paint him as too much of a villain--or too much of a hero. It is hard to tell which he fears more.

"Sometimes I have a feeling that he's gone overboard, doing more for the government than he should or had to," said Nestor Ratesh, a journalist now with Radio Free Europe who knew him in Romania. "But it's the end result that counts--but perhaps not when speaking about a rabbi, for whom the end should not justify the means. It is a tough question: How much compromise is too much? The fact is that he has saved and improved lives.

"His main fault is perhaps that he doesn't always know where the balance is between his loyalty to the regime and his service to the community--where one ends and the other begins.

"He is a great man, and history will show him as one of the most important figures of Romanian Jewish history, because he presided over the emigration of over 90 percent of the community. That didn't happen in any other East European country.

"He gives himself good grades for proving that Jewish life is possible under a communist regime. He argues that Jews manage to survive and adapt themselves to all types of regimes and social orders, and thus should be able to adapt to communism as well. But what his example really proves is that the best thing a communist government can do for Jews is to let them leave."