For the past decade in Cambodia -- during its bloody civil war, the savage revolution headed by Pol Pt and its current occupation by Vietnamese troops -- only one leading Cambodian figure held out for a "third force" to rescue his country from disaster. Finally, to his relief, Washington has decided to listen to Son Sann.

Son Sann, 69, says he is here this week to explain "to the American people so they don't confuse Cambodia as they have become confused over El Salvador." Immediately, he wants U.S. money for an international propaganda campaign and for food and medicine for the 100,000 Cambodians, living along the Cambodian border with Thailand, whose allegiance he claims.

Additionally, Son Sann told State Department officials, he wants arms -- preferably from a U.S. ally in Southeast Asia -- to build an army equivalent to the 30,000-member Khmer Rouge force. In that way, he said the interviews, he can force the Khmer Rouge to form an anti-Vietnam coalition in which his non-communist Khmer People's National Liberation Front holds most of the power.

It is not the first time Son Sann has tried to gain outside support for the middle road for his country. A French-trained financial expert -- the quintessential elite liberal who owed his position to his skills as a technocrat as much as a politician -- Son Sann for 24 years held key positions under Prince Norodom Sihanouk, including governor of the National Bank, minister of finance, minister of foreign affairs, and for a brief period, prime minister when the neighboring Vietnam War threatened to engulf Cambodia.

After the war moved to Cambodia, Son Sann found himself in the lonely position of supporting neither side. As pressures behind Cambodia's civil war gathered in the late 1960s, he openly criticized both the leftist peasant movements and the right-wing military groups.

When war broke out in 1970, Son Sann was without a political base and went into exile in Paris. He traveled back and forth from France to Cambodia, trying unsuccessfully to forge a compromise between the Khmer Rouge under the titular leadership of Sihanouk in Peking and Lon Nol and his American-supported troops in the Cambodian capital of Phnom Penh.

At the U.S. Embassy in Phnom Penh, he privately pleaded in vain for the United States to drop support of the corrupt Lon Nol government and search, instead, for a negotiated settlement of the war.

When the gruesome chapter of Cambodia's history under Pol Pot commenced with the Khmer Rouge victory in 1975, Son Sann, still in Paris, embarked on a crusade against the enforced labor, malnutrition and executions that killed more than a million Cambodians in three years.

Son Sann's general association of overseas Cambodians, first attracted the attention of the Vietnamese, who were scouting for an alternative to Pol Pot. According to European diplomatic sources, discreet overtures were made but rejected. in January 1979, the Vietnamese overran Pol Pot's Democratic Kampuchea and installed as his replacement Heng Samrin.

Still clinging to his "third force" philosophy, Son Sann had gathered a respectable number of supporters among the international Cambodian refugee communities, although he represents only a fraction of Cambodia's 6 million people.In mid-year he moved from Paris to a base inside Cambodia, just over the border of Thailand.

Although he has no effective army, Son Sann, according to a State Department study in August by Cambodian scholar Stephen Heder, emerged as the only "concrete alternative in the running" to take advantage of Cambodian opposition to the Vietnamese occupation and the Heng Samrin government. Pol Pot's Khmer Rouge, an effective army of at least 30,000 soldiers, lacks a popular base, an obvious result of the barbarism of his rule.

The Vietnamese apparently grasped the potential of Son Sann's appeal before Washington did. Last year their troops struck at his camps along the Thai border -- not the militarily superior Pol Pot bases -- and encouraged or intimidated half the Cambodians to return with them inside the country. Heder noted that the Vietnamese can continue to checkmate Son Sann by their overwhelmingly superior military force of 200,000 and an increasingly improved Cambodian economy and administration under Heng Samrin.

Although official sources said Washington has promised the food aid Son Sann needs, the political road he faces is more complicated. The United States and the Association of Southeast Asian Nations (ASEAN) believe he must find a way to form a coalition in which he would maintain political superiority to the Khmer Rouge, while insuring that the Khmer Rouge Army continues active resistance against the Vietnamese. But Son Sann, mindful of how the Cambodians dread Pol Pot, is asking the United States to help him resist ASEAN pressures to enter into a quick alliance with Pol Pot.

Although Son Sann won the intitial blessing from the Reagan administration for his endeavors and theb beginning of congressional support in Washington this week, the picture on the ground in Cambodia is not so clear.

As Heder noted in his State Department study, many Cambodians under Heng Samrin's rule may be willing to give surreptitious aid to Son Sann's guerrillas. But it is less certain whether they would sacrifice their lives -- now beginning to return to normal for the first time in more than a decade, albeit under the Vietnamese -- for an unproven "third force" whose political philosophy may be more to their liking but would require them to fight yet one more war in the name of liberation.