The exiled Nicaraguan political leaders tied to guerrillas fighting to overthrow their country's leftist Sandinista government decided last month to open up their long-secret war to the public partly out of fear that the covert backing of the United States would be cut the moment that they began to make serious headway in their fight.

Speaking privately, several of them expressed deep reservations about the covert support from Washington--"the invisible hand," as some call it--that has helped create their movement but also could be used to destroy it. They say that a point has been reached, or soon will be, at which there is no turning back and the war cannot be stopped.

They are deeply bothered, however, by the example of the Bay of Pigs invasion against Cuba more than 20 years ago, when U.S. promises and understandings with the rebel force were broken, in their view. More recently, they look with consternation at the example of the Kurds in Iraq supported by the shah of Iran and the United States, then cut off when limited political gains were achieved.

By allowing two reporters to travel with their troops inside Nicaragua, the leaders of the Nicaraguan Democratic Force hoped to establish themselves as a credible fighting force with real potential to overthrow the Sandinistas and to dispel the idea they are mere agents of U.S. policy or desperate exiles with a dangerous pipe dream.

The Democratic Force leaders were interested in establishing that the beachhead in Nicaragua was already in place, a fact borne out by our observations, and that what is going on now is not the beginning but the continuation of a serious war.

"The Bay of Pigs was an invasion on an island and very limited," said Adolfo Calero, a member of the directorate of the Nicaraguan Democratic Force, speaking from his home in Miami. "This the military action of his organization goes hundreds of kilometers inside the country. The Sandinistas could not eradicate it."

The leaders of the counterrevolution are frankly puzzled by an U.S. policy that supports them covertly, asks them to die, but through such measures as the Boland amendment in Congress expressly prohibits their ultimate goal of overthrowing the Sandinistas.

Calero attempts to define his group's struggle in terms of the U.S. law. "Some of these people are fighting for their lands, their homes. It's a defensive action, you could say."

But other Democratic Force leaders, speaking privately, are bitter about the ambiguities of their struggles.

"I would like to know where they the Reagan administration stand," said one rebel leader instrumental in setting up our trip. "Why must this be a covert thing if they are for us.

"I hate the American approach," he said, sitting in a Central American coffeeshop. "They always talk about body counts and money. They say if you don't behave we won't give you money, that if you don't do well by July we'll have to see what you get between then and November. But we are not money-makers. We are not here for money.

"They don't know what they're going to do, I think. It's like 'put pressure and see what happens. Let's do something here and see what happens there.' I don't think they have a policy."

This particular Democratic Force leader is one of the most moderate of the counterrevolutionaries and is rumored to be among the potential leaders of any government they might establish. Part of his appeal is his sensitivity to the human issues involved in the war.

"When we are in the States we are not supposed to say we are fighting to overthrow the government of Nicaragua. It's that kind of mental gymnastics," he complained. "Someone says, 'You are not to say you are fighting to overthrow the government of Nicaragua.' You should say you are fighting to democratize Nicaragua."

By opening the war to public view, the leadership of the Nicaraguan counterrevolution hopes that Washington publicly will make a decision one way or another.