With Vietnamese occupation troops and Communist Khmer Rouge guerrillas locked in a military stalemate, anticommunist Cambodians and their Southeast Asian backers once again are trying to build up a so far elusive "third force" as part of an eventual solution.

The cornerstone of the latest effort is an extended fund-raising tour of Western countries by former Cambodian prime minister Son Sann and other leaders of the Khmer People's National Liberation Front, the largest component of the noncommunist "third force" that is also known as the Khmer Serei or Free Khmer.

Backed by what amounts to a public appeal from Singapore for Western military assistance to the noncommunist resistance, Son Sann plans to set off tonight on a tour of the United States, Canada and Europe. As the price for that backing, Son Sann reluctantly has had to move closer to joining a loose coalition government with the Khmer Rouge and former Cambodian head of state Prince Norodom Sihanouk, a grouping designed to put greater pressure on Vietnam to withdraw its troops from Cambodia.

While Son Sann's group still is no match militarily for either the Vietnamese Army or the Khmer Rouge, it has developed into a well-organized political force, according to Western diplomats. In addition, it appears to be building a military infrastructure to support future growth.

At camps such as Banteay Ampil in northwestern Cambodia just across the Thai border, the Khmer People's National Liberation Front is turning out thousands of civilian cadres in a broad campaign of "political warfare" against the Vietnamese-installed government in Phnom Penh.

On the political front, according to relief officials and Western diplomats, the organization has made far greater gains than the Khmer Rouge, whose popular support is negligible because of the brutality of its rule from 1975 to 1979.

The gains have coincided with deteriorating security in some Vietnamese-controlled areas of Cambodia in recent months, according to Western and Cambodian travelers from Phnom Penh.

The front "has to be reckoned with right now politically," a Western diplomat said. He cited refugee reports of Khmer Serei political activities in the Cambodian interior. "They're organized and they're out there," he said. "They do have a network."

According to Western military sources, most of the front's estimated 8,000 to 9,000 fighters are used for the defense of its Banteay Ampil complex across the border from the Thai village of Ban Sangae about 35 miles north of Aranyaprathet. They said that while the front's military commander, Dien Del, has an active and systematic training program, the front's forces rarely combat the Vietnamese or Phnom Penh government troops.

"Dien Del is scared to take too many casualties when he's trying to build up his force," one Western defense attache said. Currently, the 30,000 to 40,000 guerrillas of the Chinese-backed Khmer Rouge still do most of the fighting against the Vietnamese, who have as many as 200,000 troops in Cambodia.

In private, the Chinese at times have been scathingly critical of the front, charging that it does not do enough fighting and has not effectively used the weapons that Peking has already delivered to it, the military attache said. He said Thai officials have also criticized the front, accusing its members of being more interested in smuggling and leading comfortable lives than in fighting the Vietnamese.

While there is some truth to this, the Western attache said, "on the other hand, I'd see it differently if I were Dien Del. I would want to fight to the last Khmer Rouge, too."

He added that on the basis of personal observations, the front seemed to have "quite good defenses" and would not "be a pushover" if the Vietnamese attacked. "I was quite impressed with their organization," he said.

Evidence of this organization can be seen at camps such as Banteay Ampil. During a recent visit there, young recruits were drilling with Chinese 75mm recoilless rifles and AK57 machine guns supplied earlier this year.

Spread around the sprawling jungle camp are bunkers, munitions storehouses, barracks and a military hospital built of wood and thatch. Two small settlements at the camp, which is off-limits to foreign relief workers, consist of neat rows of thatch-roofed huts reserved for soldiers and their families.

Among the training programs at the camp are a six-month course to turn out noncommissioned officers and a three-month course for company commanders. Another program has just been started to train civilian teachers and medical aides, and an agricultural course is planned.

Along with grade school students and soldiers, the trainees lined up in neat ranks for a ceremonial meeting with Son Sann during a visit he made there in the fall. Also present were about 20 children, from 10 to 14 years old, wearing camouflage uniforms. All orphans, they are enrolled in a "pre-cadet school."

The organization of the front's camps and resistance forces is largely the work of Dien Del, a tough 49-year-old former general in Cambodia's pre-1975 republican army. He is considered one of the few officers to distinguish himself in that army's losing battle against the Khmer Rouge, who seized Phnom Penh in April 1975 and held power until ousted by Vietnamese troops in January 1979.

Since the beginning of the year, Dien Del has moved ruthlessly to eliminate -- with help from the Thai military, according to some accounts -- the Cambodian warlords who once controlled most of the border area's noncommunist camps.

Clashes between different factions for control of black market activity on the border have become much less frequent in recent months, and a measure of order and stability has come to the seven camps now run by the front.

"Dien Del has brought the noncommunist area under control," said a Western diplomat in Bangkok. "Under his aegis communities are developing."

One of the largest front communities is at the Rithisen Camp, which was recently renamed after a legendary Cambodian hero.

There, 513 young men and women are currently enrolled in the political warfare school to study history, geography, psychology, Khmer culture, the politics of the front and "the techniques of political warfare," according to Hing Kunthon, one of Son Sann's top aides. In addition, he said, the students receive first-aid training and do manual labor in preparation for their infiltration into the Cambodian interior.

According to Thou Thonn, the director of the political warfare school, about three-fourths of the graduates are sent to the interior. The rest remain in the border area, some attached to the front's army as political advisers.

Front officials said the main functions of those sent into Vietnamese-controlled areas of Cambodia are to gather intelligence, organize "subversion" and wage "psychological warfare."

Between 5 percent and 10 percent of those sent in are caught, officials said, and a few hundred front members are currently held in Cambodian jails.

Western and Asian diplomats in Bangkok believe the front's emphasis on training political agents and other civilian cadres could become increasingly important in the future. If there is to be any solution to the Cambodian issue, they say, it is more likely to be a political than a military one.