The Nicaraguans who once fought for their country's revolutionary Sandinistas and now fight from exile against them say their efforts are being undermined by U.S. backing of those who supported the deposed Somoza government.

"We can take one enemy at a time," as one moderate exile leader put it recently. "But now we are fighting against the Sandinista National Directorate and the U.S. State Department."

These exiles, led by former Sandinista commander Eden Pastora and ex-members of the revolutionary junta Alfonso Robelo and Arturo Cruz, say they are trying to form a united opposition to the current Sandinista leadership that will be both "democratic" and "revolutionary."

But they say the Reagan administration has decided instead on covert backing for former members of the rightist Somoza government's National Guard and has moved to push more liberal anti-Sandinista forces out of the picture.

Specifically, they charge that Washington has encouraged their exclusion from Honduras and all but stifled their activities in Costa Rica, the two countries bordering Nicaragua on the north and south.

"The only thing that opens and closes doors these days in Honduras and Costa Rica is the United States," said one frustrated member of the Pastora group.

In the explosive, Byzantine world of competitive plots and counterplots aimed at overthrowing the Marxist-oriented Sandinista leadership, Pastora and his allies see themselves increasingly undercut and discredited.

By the same token, while their movement reportedly has provoked some divisions within the Sandinista ranks, it also has intensified bitter splits among various other factions hoping for the Sandinistas' downfall.

Although Nicaragua's revolutionary government tends to lump all its opponents together as "counterrevolutionaries," the exiles are divided by several small differences and split down the middle over one big question: the role of Nicaraguan ex-Guardsmen who for almost 50 years formed the foundation of the hated Somoza family dictatorship and waged a bloody but unsuccessful fight to keep it in power during the 1979 insurrection.

Two thousand or more of these soldiers escaped into Honduras in the last days of that war and have since regrouped into what is, in military terms, the most potent Nicaraguan force marshaled against the Sandinistas.

The ex-Guardsmen and other conservative anti-Sandinista opponents have been brought together recently into an umbrella organization called the Nicaraguan Democratic Force, led by exiled businessman Jose Francisco Cardenal.

Pastora's people say Cardenal is nothing but a figurehead and that he is getting more than tacit U.S. support because Washington wants a "docile force."

Leonel Poveda, Pastora's top aide, said he thinks Washington "is a little afraid" that Pastora will grow too strong. "My impression is that they don't think they can handle our man and the people around our man," Poveda said in a telephone interview.

For his part, Cardenal said from Miami in a separate telephone interview, "We will carry out the struggle against the Sandinistas with, or without, or against Pastora. We don't want a solution that would last one or two years and then have to fight again against a new government."

"We want to liberate Nicaragua from the Communists' system, not just their leaders," Cardenal added, suggesting that Pastora would present just a slightly different brand of radical socialism. "He attacks American imperialism and praises Castro and Qaddafi. Anybody who speaks like that is our enemy."

Meanwhile, the Nicaraguan government does what it can to intensify the divisions among its opponents.

The Sandinistas helicoptered reporters and diplomats into the battered Nicaraguan village of San Francisco del Norte two weeks ago to witness the grim aftermath of an attack by a group of Cardenal's partisans. Sandinista officials there said the insurgents were heard to shout "Long live the National Guard! Long live Eden Pastora!" as, allegedly, they gunned down Sandinista militiamen.

Pastora, the highly emotional hero known during the revolutionary war as "Commander Zero," recently placed an advertisement in Costa Rican newspapers rejecting any suggestion that he might join with Cardenal. He claimed that to do so would be "to mix progressive revolutionary forces with the criminal mummies and gorillas of the National Guard."

"As one more proof of my loyalty to the people of Nicaragua and my revolutionary firmness," said Pastora, he would "dissolve" his self-styled Sandino Revolutionary Front as a political-military organization "as long as the genocidal National Guard exists as an armed institution in the north of Nicaragua."

Poveda and other friends of Pastora now say the dissolution is more of a "temporary freeze" to see what happens.

A secret meeting was held in Costa Rica last weekend that included, according to exile spokesmen, Pastora, Robelo, Cruz, Fernando Chamorro and Brooklyn Rivera, a leader of one of the rebellious Miskito Indian factions in Nicaragua's Zelaya Province.

The meeting's purpose was to develop a new "democratic alliance," said spokesmen for Robelo and Pastora, but little of substance emerged.

Chamorro and his brother Edmundo, who run a small military organization that split off from the old Conservative Party during the Somoza dictatorship, express some willingness to compromise with the ex-guard factions to build a substantial fighting force.

But Pastora and his people say military action against the Sandinistas should be a last resort, and that before it is attempted, the nine Sandinista leaders on the all-powerful National Directorate should be undermined politically inside the country and diplomatically outside.

Pastora's people say the ex-Guardsmen are a political and diplomatic disaster for any movement against the current Nicaraguan leadership, because they are incapable of garnering broad international support and utterly without the confidence of the Nicaraguan people--who rose up against the National Guard in the first place.

Spokesmen for Pastora's group said in telephone interviews that they believe the ex-Guardsmen are no match militarily for the 25,000-strong Sandinista regular Army and tens of thousands more in militias and reserves.