With voting expected in New York this week on who -- if anyone -- should represent Cambodia at the United Nations, the country's rival governments and their allies abroad have launched energetic campaigns of diplomacy and propaganda to seek support for claims.

Both the Khmer Rouge, who presently occupy Cambodia's U.N. seat, and the Heng Samrin government, installed in Phnom Penh by Vietnamese troops 20 months ago, see the world body's stamp of legitimacy as an important psychological weapon in the conflict.

Diplomatic analysts here believe that the Khmer Rouge, despite their reputation for brutallity while in power, will gather enough votes -- including that of the United States -- to keep a General Assembly seat for another year, but probably not longer than that.

[In Washington, Secretary of State Edmund Muskie announced Monday that the United States would vote to allow continued U.N. seating for the Pol Pot representative.]

Rejection of foreign aggression has been the theme of Khmer Rouge pleas for support. The ousted government this month announced it had sent letters to 100 countries deatiling Vietnamese transgressions in Cambodia.

This summer, that government's social affairs minister, Ieng Thirith, argued the case on a tour of African countries (considered by many to be swing votes), while a Khmer Rouge delegation attending a special U.N. session in New York this month lobbied ambassadors there.

Pol Pot's main allies, China and the Western-oriented Association of Southeast Asian Nations, have provided their own diplomatic support. Chinese embassies put added pressure on African governments, while Cabinet ministers from ASEAN member Singapore covered Middle Eastern countries and a recent New Delhi conference of Commonwealth nations from Asia.

Khmer Rouge Prime Minister Khieu Samphan, meanwhile, has hosted a stream of Western journalists at a camp close to Cambodia's northern border. His message: communism is no longer suitable to Cambodia's needs.

One group of reporters was told Khmer Rouge forces would stage a major attack before the General Assembly convened, apparently to refute Vietnamese claims that Pol Pot's troops are all but wiped out.

Last week Khmer Rouge Radio claimed its troops had killed or wounded 79 Vietnamese in and around Phnom Penh last month. Foreign aid agencies with personnel in the capital however, said they were not aware of fighting in the capital.

Heng Samrin, assisted by Vietnam, the Soviet Union and other Eastern Bloc countries, has launched an opposing effort, arguing that Pol Pot's government was among history's most brutal and that Heng Samrin now holds effective control and support of most of Cambodia's people.

Phnom Penh Foreign Minister Hun Sen made an extensive foreign tour of his own this summer spending time in Nicaragua, the Congo, Angola and Eastern Europe. In countries the envoys cannot visit -- only about 30 countries currently recognize the diplomatically isolated government -- Heng Samrin's case is pushed by Soviet and Vietnamese diplomats.

Radio Phnom Penh this weekend said long-promised national elections would be held early next year, a promise apparently geared to the U.N. vote. Earlier the radio broadcast reports of a newly discovered mass grave and claimed the Khmer Rouge killed nearly all of Cambodia's 700,000 Moslems during their year in power.

Recently the U.N. High Commissioner for Refugees reached agreement with Phnom Penh to assist Cambodains who had returned to their homes. Some analysts feel Heng Samrin wanted de facto recognition from another U.N. agency as much as the aid itself. (UNICEF), the World Food Council and the Food and Agriculture Organization are already represented in Phnom Penh.)

Last week, the Heng Samrin government cabled the United Nations demanding to be seated in place of the Khmer Rouge, setting in motion what is certain to be a major credentials fight.

If one of nine member states in the U.N. Credentials Committee objects to the Khmer Rougehs continued presence, the issue goes to a vote in the General Assembly.

In a similar battle last year, 71 countries voted to contunue seating the Khmer Rouge, 35 voted against them and 34 abstained.

Diplomatic analysts in Bangkok generally expect the Khmer Rouge to survive the coming vote, though with a far thinner margin. Many of last year's supporters will abstain, it is predicted, due to fading memories of the Vietnamese invasion and new disclosures of Pol Pot's brutality.

At the same time a few more countries -- Nicaragua, for example -- will join the vote against Pol Pot.

If these predictions are wrong and Pol Pot is voted out, Vietnam or some other country can be expected to introduce a resolution to seat Heng Samrin.Most analysts, however, see almost no chance that such a move, if introduced, would be successful, given Heng Samrin's poor record at winning diplomatic recognition.

Heng Samrin's defeat would leave the Cambodian seat open -- a solution favored by many countries -- until a more acceptable government was found. Israel and its allies are reportedly wary, however that such action would set a precedent for a similar move against the Jewish state.

Diplomats here expected the United States to vote for the Khmer Rouge, as it did last year.

Reluctant to alienate China and ASEAN or to acquiesce to the extension by force of Soviet-Vietnamese influence into Cambodia, the United States will put aside its misgivings over the Khmer Rouge's record on human rights, diplomats believe.

Though debate continues within the Carter administration, the essential United States position is that voting for Pol Pot upholds the best interests of peace. It will intensify pressure against the Vietnamese to negotiate and to create a government acceptable to all sides, it is said.

The ideal Cambodian government would be neutral, restoring the country's role as a buffer between Vietnam and Thiland, American policymakers say.

At the same time, they believe that the Khmer Rouge are the only military force capable of bleeding the Vietnamese into negotiation. Anticommunist Khmer Serei armies are too small and prone to infighting to inflict the necessary wounds.

Critics of U.S. policy question how the Khmer Rouge would be kept from dominating a future government if their soldiers were responsible for bringing it about. How, it is asked, could neutralist elements, such as former head of state Norodom Sihanouk, play a serious role if they had no military backing of their own?

Suggestions that the West come to terms with Phnom Penh, then attempt to loosen its dependency on Vietnam and the Soviet Union through aid and diplomacy, are rejected as naive appeasement that could lead to future conflict between Indochina and Thailand.

ASEAN diplomats have drafted a peace plan of their own, to be put on the table after the U.N. credentials vote. It will apparently favor an international conference on Cambodia and U.N.-supervised elections.

[China announced Monday, however, that it would require Vietnam to begin troop withdrawals and set up a timetable for future pullouts before any international conference could take up the question of Cambodia, Agence France-Presse reported from Peking.]