Expelled from sanctuaries in Indochina and receiving only token aid and encouragement from its former patron, China, the Communist Party of Thailand today finds the momentum of its 15-year guerrilla war against the Thai government has been broken.

Although the party's estimated 10,000 soldiers continue attacks in three rural regions of the country, there is trouble in the ranks -- official Thai figures show that defections tripled last year and government casualties dropped to 472 dead from 654 in 1978.

Communist setbacks result from the party's decision to side with Peking in the Sino-Vietnamese conflict, which flared last year when Hanoi's army swept across Cambodia and routed the Chinese-supported Khmer Rouge government.

As Vietnamese armor and infantry moved westward, Thai communist bases and political schools in Cambodia were evacuated into Thailand.

At about the same time, the pro-Vietnamese government of neighboring Laos ordered Thai party cadres and soldiers to vacate about 20 important camps in its territory.

These losses, however, brought no increase in Chinese aid. Instead, Peking reduced arms supplies, shut down the party's powerful radio station in southern China, and began counseling Thai communist leaders that Soviet and Vietnamese expansion, not attacks from the Thai government, were the most important threat to the Thai revolution.

Weakening these long-standing ties with the Thai party was China's end of a complex secret deal with Bangkok, many Indochina specialists believe. In return, the Thai Army has helped channel Chinese munitions to Khmer Rouge guerrillas harassing the Vietnamese in Cambodia, the specialists believe.

Old-guard revolutionaries who dominate the 24-member Thai Central Committee have accepted China's new strategy with little dissent, although it appears to have compromised the party's slow but steady expansion in rural communities.

But Thais with party contacts report debate and disillusionment among some party members, particularly students, intellectuals and labor leaders who joined the insurgents in 1976 after a bloody coup ended three years of civilian rule in Bangkok.

Several thousand persons entered the jungle that year, leading some analysts to announce that the party had reached a turning point that gave it a foothold in the cities and backing from a diverse collection of social classes. Soon the party unveiled a new united front organization called the Coordinating Committee for Patriotic and Democratic Forces. It united with representatives of the Thai Socialist Party and student, labor and farmers' groups.

Many of these persons were assigned to propaganda duty. Working at the Thai party headquarters in northern Laos, 120 persons turned out one English-language and two Thai publications, as well as radio programming. They had a telex link to the studios of the Voice of the Thai People, which broadcast from Kunming, China.

Accustomed to the give-and-take of debate on a university campus, many students were soon chafing under the stern theoretical directives issued from Central Committee members, some of them ethnic Chinese who speak Thai with a Chinese accent. They were frustrated by party activities, such as the mandated debate and criticism of the "Gang of Four" -- which many Thai students considered a purely internal Chinese matter.

Students also argued that the party should accelerate its efforts to organize covertly in the cities.

The rigors and bordeom of life in the isolated camps and the intellectual control exercised by the party led many newcomers to seek asylum in countries such as Sweden -- where about 200 are said to have shown up -- or to drift back to the cities.

Former student leader Saeksan Prasertkun, who turned himself in to the government Oct. 1, spent much of his time in the jungle as a foot soldier, carrying an AK-47 rifle and living in a pup tent. Party cadre wanted the headstrong Saeksan to learn obedience and humility.

meanwhile, the party took care to keep the newcomers from positions of influence. Few became full party members. The party's armed forces, recruited primarily in rural villages, and its corps of political workers remained firmly under the control of the old guard.

Thus when Vietnam invaded Cambodia, the party fell immediately into line with the Chinese and Khmer Rouge. The Oct. 14 and Oct. 16 cadre schools, named for days in 1973 and 1976 when Thai troops broke up major student demonstrations in Bangkok, were evacuated from Cambodia immediately.

Thai sources say the Khmer Rouge quickly asked the party to assign 500 persons to help in border operations against the Vietnamese. The request was granted.

At approximately the same time, Laotian military officers showed up at the party's headquarters there and announced that everyone had to leave. Altogether, about 20 Thai party bases in Laos were lost.

Closure of the truck routes through Laos -- which for years had brought rifles, ammunition, rice, canned goods and even contraceptive pills to the Thai insurgents -- created serious problems. It meant that the only access from China was a dangerous weeks-long journey on foot through parts of northern Burma controlled by anticommunist separatists.

However, China appears to have made no effort to find better supply routes.

At a June 1979 meeting in Peking, Li I Man, a ranking Chinese official dealing with party-to-party relations, explained to skeptical Thais the need for a new approach.

Vietnamese aggression was the primary threat to the party's survival, he said, and since the Thai Army already was helping supply the Khmer Rouge, perhaps the Thai party should work out a truce with Bangkok.

In July interviews with party publications, party leaders came firmly out on the side of China and proposed a cooperative effort of "all strata" of Thai society against Vietnam.

Also that month, the party's radio station went off the air -- "temporarily," the announcer said. It has not been heard from since. Thais who had been assigned to a station that also broadcast programming for pro-Chinese communist groups in Malaysia and Burma were put to work in a newly formed news agency whose mimeographed sheets reach only a handful of people.