AT A SANDINISTA TRAINING CAMP - According to President Anastasio Somoza, the Sandinista National Liberation Front is a gang of foreign-supported Marxist terrorists who want to turn Nicaragua into a base for international communism to conquer Central America.

Even the Carter administration - whose sympathies now lie more with Somoza's opponents than with his 44-year-old dynasty whose U.S. roots have become an embarrassment, - fears that the Sandinistas seek to build another Cuba in Nicaragua.

Sandinista political leaders interviewed here recently denied that they are Marxists. They denied that they want Cuban-style communism in Nicaragua. Instead, they said, they are fighting for a "new Nicaragua" that will be a "pluralistic democracy" built on the ashes of the destroyed Somoza dictatorship.

While U.S. congressional conservatives remind each other and Carter that Fidel Castro, in the years before his takeover in Cuba also denied he was a communist, the Sandinistas say the proof of their sincerity is in their past deeds, their current actions, and in the future they plan for Nicaragua.

"Carter's next election is in two years," said Eden Pastora, the Sandinista military commander. "Within that time, we can prove ourselves, prove that we are not communists." While they do not ask for Carter's direct support of their own efforts, they ask, the Sandinistas said, for the end of all direct and indirect military and economic relations with the Somoza government by the United States and its allies.

"In two years," Pastora said, "we will already have had elections, and Carter can prove to the American people that he didn't make a mistake with us. That's the best proof we can offer."

During most of their 17-year history, the Sandinistas did not bother to argue politics with their internal enemies and international skeptics. There were few interested listeners, in any case.

Lodged in mountain hideouts from which they launched largely ineffective attacks against remote National Guard outposts, the Sandinistas were not viewed as a determining factor in the court of Nicaraguan events.

In the past year, however, beginning with a strong offensive against the National Guard last October and culminating in the August attack on the National Palace in Managua that sparked a civil war, the Sandinistas have emerged as a potent fighting force with massive support from the Nicaraguan population and the at least tacit backing of the largely conservative anti-Somosa political and business majority.

One of the main problems the Sansinista leadership has in convincing even those already against Somoza of their ultimate goals is the origin of the organization. Its inspiration was the Cuban Revolution. Which coincided with the Sandinistas- own beginnings - as with the beginnings of similar movements throughout Latin America nearly two decades ago.

"Many of our leaders were influenced by the Cuban revolution," said one Sandinista, "because it made them believe such a thing was possible in Nicaragua." At various times, Cuba has given asylum, and small amounts of training, to a number of Sandinistas.

In the past, said Daniel Ortega Saavedre, 36, a founder of the Sandinistas and a member of the organization's current national directorate, "there were those who wanted a more radical change to follow the fall of Somoza."

"Now." Ortega said, "we see that this is not possible for a number of geopolitical reasons, and our position now is for the imposition of a democratic government . . . a government that joins together all those who fought against Somoza."

Ortega is a youthful-looking, intense man who began fighting against Somoza as a teen-ager. One of his brothers, Humberto, is also a member of the directorate. Another brother, Camilo, was killed in a recent Sandinista-National Guard shootout.

Ortega and Victor Manuel Tirada, a sleepy-eyed, 38-year-old Mexican who came to Nicaragua and helped form the Sandinistas 20 years ago, spoke to a reporter in the home of one of their sympathizers in a Central American city.

While they seemed reluctant to discuss internal Sandinista divisions, they and other members in interviews described the organization's political genesis as a transformation from a rigid commitment to the "dictatorship of the proletariat" to a recognition by the bulk of them that a socialist revolution would not succeed in Nicaragua.

That realization resulted in a three-way split in the Sandinista organization among the mountain fighters, the Marxist propagandists, and the Insurrectionists - by far the largest and most active group.It is the third group, whose national leadership for all practical purposes runs the Sandinista National Liberation Front, that is responsible for attacks during the past year.

While the Sandinista movement has been a factor in Nicaragua, albelt often dormant, for as long as the Somozas themselves have been around.

The guerrillas take their name from Gen. Augusto Cesar Sandino, who formed his own guerrilla movement in the 1920s to protect U.S. Marine occupation of Nicaragua. In 1934, after the Marines had gone, leaving Gen. Anastasio Somoza Garcia, the current president's father, in charge of the Marine-formed National Guard, Sandino came down from the mountains to make peace with the Nicaraguan government.

His subsequent assassination by National Guard officers is generally believed to have been ordered by Gen. Somoza, who feared the threatening popularity of the "bandit" who had eluded the Marines for so long.

While the Somozas and their supporters remember Sandino as an arch-criminal, many Nicaraguans consider him a patriot.

"Sandino's program was fundamentally a nationalist one," Ortega said. "Our job is to unite with that long-ago period that was cut short."

It was not until 1962 that a scattered assortment of Nicaraguan guerrilla groups began to pull themselves together to form the Sandinista Front under the leadership of Carolos Fonseca Amador. Until recently, the small group of Sandinistas has had little success, first against the government of Luis Somoza, the early president's eldest son, then a scattering of puppet presidents, and now the current President Somoza.

"It's a shame that the United States doesn't understand this revolution," Ortega said, "because that was so must a part of their own history. They can say whatever they want about us, but the truth is found in deeds.

"We are, and have always been, for one very calculated goal - the fall of the Somoza dictatorship. We want the installation of a popular, democratic government that responds to the people's needs, that gives the people work, that gives land to the peasants, and health services."

All of these items are spelled out in a 25-point Sandinista program that includes, in addition, the immediate confiscation of all property belonging to Somoza, members of his family and high National Guard and ruling party officials.

The Sandinistas also say they plan to nationalize "national resources," including mines and forests, to form a new national army, and to establish relations with "all countries of the world" compatible with Nicaragua's economic development.

The Sandinistas say they will form their own political party after Somoza is defeated, and feel certain that they will have no trouble winning office.

Whether they mean what they say, or as Somoza has warned for the past 17 years - are communists in ill-fitting democratic clothing, cannot be proven.

The Sandinistas maintain that, despite the fact they are still massively outgunned by the National Guard, the eventual victory of which they are certain will be a military and not a political one.

They say they neither support nor intend to sabotage current U.S.-sponsored mediation between Somoza and his political opponents. But they do not hide their longtime antagonism to and suspicion of both the huge northern neighbor they believe has propped up the Somoza family and the Nicaraguan groups that only recently came out on their side.

"We will respect any agreement that benefit the people of Nicaragua." Ortega said. "And the only agreement that meets that qualification is one that includes the complete eradication of Somoza."