EIGHTEEN MONTHS ago, democracy replaced dictatorship in the Philippines and the United States was widely heralded for its contribution to that significant transition. Now, as South Korea teeters between political freedom and repression, Washington will again play a critical role.

There are of course significant cultural and historic differences between the Philippines and Korea. But, from the perspective of American policy, there are compelling parallels. The United States can apply in Korea what it learned from the Philippine experience -- what it did right as well as wrong. If, however, Washington is going to replicate the success it had in the Philippines, it will have to act decisively in favor of democracy and not waver in the face of ingrained tendencies to stay with a dictator because he is pro-American.

There are few risks involved for the United States from hanging tough in Korea. No Communist insurgency threatens to step into a vacuum. The economy is robust and no democratically-elected leader is going to deny Koreans their substantial economic gains. Nor would the opposition seriously consider tossing out the American troops, which protect the South Koreans from the Communist North.

Nevertheless, a State Department spokesman said recently that it "would be wrong to talk about our launching any initiatives" because what was happening in Korea was "an internal Korean matter." But initiatives are needed, and since no one seriously believes that the United States is going to remain neutral and above the fray in Korea, the question is how Washington is going to intervene.

The Reagan administration's first move must be to abandon quiet diplomacy. The experiences of the Carter and Reagan administrations with Ferdinand Marcos illustrate the futility and frustration of that approach.

The Carter administration wanted Marcos to lift martial law and hold honest elections. He did neither, continuing his unbridled accumulation of wealth and power. The Reagan administration pressed Marcos quietly for reforms -- economic, military and political. But the policy couldn't work: It was asking Samson to cut his own hair. Although the Reagan administration has taken credit for the outcome in Manila, what eventually led to Marcos' fall was not so much American policy as it was first his own blunder -- if he hadn't called "the snap election," he would almost certainly still be in the Malacanang Palace today -- as well as the Filipino people in the streets.

As for South Korea's President Chun Doo Hwan, he might adopt some of the reforms the Americans have been quietly urging. But as Wall Street Journal reporter Joseph P. Manguno observed from Seoul last week, "substantial concessions by the government to advance the process of democratization are unlikely, because they would compel Mr. Chun to give up his master plan for political succession and the establishment of the Democratic Justice Party, or DJP, as the core of the government."

If the United States is to move Chun and his political clique and be a handmaiden to democracy, it will require some near shouting -- figuratively if not literally -- merely to overcome the impression that the United States has tilted toward Korean dictators, much as it supported Marcos for more than a decade.

In October 1972, South Korea's then-President Park Chung Hee declared martial law -- three weeks after Marcos had done the same in the Philippines -- and there was barely a peep from the Nixon administration.

Chun was officially welcomed at the White House only two weeks after President Reagan was inaugurated in 1981.

As ambassador to Korea, the Reagan administration replaced William Gleysteen Jr., a highly regarded career diplomat who was critical of Korean dictators, with Richard ("Dixie") Walker, an arch-conservative university professor who staunchly defended Chun.

Just as American officials over the years limited contact with Marcos' opponents lest they offend the dictator, so have they generally kept their distance from the Korean opposition. During five years in Seoul, Walker ignored the opposition, met only once with Kim Young Sam and not at all with Kim Dae Jung, the de facto leaders of the opposition. During a visit to Seoul in May 1986, Secretary of State George Shultz refused to meet with Kim Dae Jung.

It was not until earlier this year that Ambassador James Lilley, Walker's successor, finally met with Kim Young Sam; last week the assistant secretary of state for East Asia, Gaston Sigur, met with Kim Dae Jung. Apparently the United States has learned an important lesson: Maintain good relations with the opposition because, as in the Philippines, they might become the government. Highest level U.S. officials must now be seen with the Korean opposition leaders, as they are with Chun and his party.

There is another lesson from the Philippine experience: Foreign policy is generally better left in the hands of the professional diplomats. Whatever credit the United States deserves for the return of democracy to the Philippines belongs first and foremost to the State Department, which along the way had to maneuver around resistance from the White House. Until the very end, Reagan and his conservative advisers, in and out of government, backed Marcos, while the diplomats were solidly in the camp of democracy.

In the Korean crisis, with the White House considerably weakened and preoccupied by the Iran-contra affair, the department should have even more room to operate. But it can't be taken for granted that the State Department will be able to repeat its Philippine success in Korea.

To begin with, the key policymakers are different. Under Secretary of State Michael Armacost, who played a fundamental role in persuading Shultz that a change of American policy was required in the Philippines, has not been as engaged by Korea as he was by the Philippines, where he had served as ambassador.

At a lower bureaucratic level, on the Philippine desk at the department was John Maisto, a career foreign-service officer whose impact on policy was disproportionate to his position. Maisto's influence was in part attributable to his knowledge of the Philippines, where he had served before returning to the United States. In Washington, he continued in Philippine affairs, spending an unusually long period of time for a foreign-service officer on one country. "There is no one the equivalent of Maisto" on the Korean crisis, said a congressional aide who has been closely monitoring Korean policy as he did the Philippines.

Perhaps the most critical difference, however, is on the ground. The American ambassador in Manila during the crisis was Stephen Bosworth, a career foreign service officer whom Shultz talked out of taking early retirement in order to take on the Philippine problem. A consummate diplomat, Bosworth did not succumb to Imelda Marcos' lavish blandishments, and he earned the respect and trust of church and opposition leaders. Though Bosworth did indeed retire recently, he is still three years shy of 50 and the administration should consider asking him to return as ambassador-at-large.

Washington's man in Seoul, Ambassador Lilley, is lacking in diplomatic experience. A retired CIA officer who joined the agency in 1951 and held posts in Manila, Beijing and Taiwan, Lilley has added to the impression that Washington backs the dictator. Last month he attended the ruling party convention and earlier, when advised that Chun was going to suspend debate on the constitution, Lilley did not protest.

One obstacle the State Department will face in Korea is the Pentagon. In the Philippines, the Pentagon was in agreement with State that continuation of Marcos' dictatorship worked against America's interest. In Korea, U.S. military officials have been reluctant to send strong signals to the Korean military that it stay out of politics and support a return to democracy. The difference in attitude can be explained in part by the anger American military officials felt at Marcos for the deterioration of the Philippine army. That deterioration fueled their concern about a communist takeover the longer Marcos continued in power. The Korean army remains strong and there is no communist insurgency there, thus diminishing any pressure for change from American military circles.

In negotiations with Korean leaders, what the United States has to say will be influential. "A Korean leader needs to be seen as doing things that are acceptable to the United States," says former Ambassador Gleysteen. It is "very damaging" to a Korean leader, he adds, to be perceived by Koreans as causing a deterioration in Korean-American relations.

Moreover, the United States has two powerful trump cards to play: trade and troops. Nearly half of Korea's exports go to the United States and if Washington is willing to use trade against Nicaragua and South Africa, it shouldn't flinch before the Koreans. As for the 40,000 American soldiers, although they are obviously important for American security, it should not be forgotten that they are just as vital, if not more so, to the defense of South Korea against the North.

Although Chun might assume that the Reagan administration would not remove the troops, he should be reminded that few ever thought Reagan would be advocating the arms-control agreement with the Soviets that he now is. In the high-stakes poker game for democracy the United States shouldn't blink, as it did regularly with Marcos.

In pressing for democracy in South Korea the Reagan administration has a number of options, some of which were tested in the Philippines.

One possibility would be to send an emissary to Chun with a strong message about the administration's position. This is precisely what was tried, at the State Department's urging, when it became clear that the diplomats were not penetrating Marcos with the message for reform. That mission failed, but the failure was in the execution, not in the concept. The messenger to Marcos was Sen. Paul Laxalt, who was chosen because of his closeness to Reagan. It was widely reported that Laxalt delivered a blunt message to Marcos, but in fact, the Marcos-Laxalt meeting was a "love fest," as an American diplomat described it. If an emissary is sent to Korea, he should in fact deliver a clear message.

Short of an emissary, high-level U.S. officials must speak out more forcefully in support of democratic change. To date, the administration has sent inconsistent signals. Last February, Assistant Secretary Sigur delivered a speech calling for democratic reforms in Korea, but Shultz has passed up the opportunity to endorse it unequivocally. Within the department there is discussion about the need for Shultz to make a speech at least as strong as Sigur's. (International PEN, the writers' organization devoted to free expression, and the World Peace Through Law organization could do their part by reversing plans to hold gatherings in Korea, where there is strict press censorship, political prisoners are tortured and there is no due process.)

The United States is on a high wire in Korea, but the precarious position is in part a result of its own past actions. Within the administration, officials acknowledge that Washington has been dilatory in developing a policy that would take into account Korea's desire for democracy. It's not too late. Korea, like the Philippines, is too important to Asian and American security for issues to be settled in the streets of Seoul -- or the councils of great caution in Washington.

Raymond Bonner is the author of "Waltzing With A Dictator: The Marcoses and the Making of American Policy."