The Reagan administration's campaign to prove that Nicaragua and Cuba are interfering in El Salvador's civil war crashed into a credibility barrier yesterday when a captured Nicaraguan guerrilla, whom U.S. officials expected to support their charges, said his earlier confessions had been made under torture and threat of death.

The guerrilla, Orlando Jose Tardencillas Espinosa, 19, who was captured in El Salvador last year, was presented to reporters at the State Department on the expectation that he would back up U.S. charges that Cuba and Nicaragua are supplying, training and directing the leftist insurgents fighting El Salvador's civilian-military junta.

Instead, Tardencillas completely recanted statements he had made in El Salvador about having been trained in Ethiopia and having been sent by Nicaragua's revolutionary Sandinista forces to help the Salvadoran insurgents. "What I said in the past was part of a fascist propaganda effort," he asserted. "I was tortured. They used physical force and scientific methods. I was batted about. I had to be operated upon for a clot in my head from the beating."

Tardencillas, who said he was brought to the United States Tuesday by two Salvadoran security officers, continued: "An official of the U.S. Embassy told me they needed me to demonstrate the presence of Cubans in El Salvador. They told me I could come here, or I could face certain death."

In using the term "they," he appeared to be talking about Salvadoran authorities, and he added: "They expect that through sensationalist propaganda they will justify their crimes . . . . They thought I would say the same things here. All those statements in El Salvador were false."

State Department officials sat in shocked silence through the interview with Tardencillas, speaking in Spanish through an interpreter to a small pool of reporters who afterward gave tape recordings of the session to the rest of the press. Later, the department issued a statement about Tardencillas that said in part:

"When we decided to make him available to the press today, we knew we were taking a chance. He had never denied being a revolutionary, and he had made clear during our interviews with him that he would make statements in support of the revolution in El Salvador.

"Nonetheless, based on information he had given us and what he had said publicly before television cameras in El Salvador, we believed it worthwhile to make him available here. Obviously, he has either lied to his interviewers and the public since his capture or he lied to the press today."

The statement ended by reiterating that "however he changed his story today, he did confirm that, as a Nicaraguan, he had commanded guerrilla forces in El Salvador."

That stubborn concluding note underscored how much the administration has invested in trying to overcome public and congressional skepticism about its controversial Central America policy. Last week, Secretary of State Alexander M. Haig Jr. was embarrassed badly when he told a congressional committee that a Nicaraguan guerrilla had been captured in El Salvador, only to learn afterward that the individual had escaped into asylum within the Mexican Embassy and claimed to be a student rather than a revolutionary.

Goaded by that incident and by mounting congressional demands for proof of Cuban and Nicaraguan subversion, the administration this week unleashed an all-out campaign to make its case through such means as a press briefing featuring blowups of aerial photographs allegedly showing a Nicaraguan military buildup and private briefings for select members of Congress and officials of past administrations.

But when the government fired off its latest shot yesterday, it misfired so badly that a big question was left as to whether the public can be induced to take seriously any future evidence that the administration might put forward about the alleged Cuban and Nicaraguan threat to Central America and the Caribbean.

In the interview, Tardencillas freely conceded that he is a Nicaraguan, born in the town of Masaya, that he had fought as a Sandinista guerrilla in the Nicaraguan civil war that in 1979 overthrew the dictatorship of Anastasio Somoza, and that he afterward served for a few months in the reconstituted Nicaraguan army. He also admitted that he joined the Salvadoran guerrillas in April, 1980, rose to command guerrilla forces in the Libertad and La Paz regions of El Salvador and was captured by government troops on Jan. 30, 1981.

However, he insisted that he had gone to El Salvador only because he wanted to help the Salvadoran people "fight the criminal activities, terrorism and human-rights abuses of the Salvadoran government." He insisted that he made his own decision to go, was not sent by the Nicaraguan government or Sandinista elements, had no contact with Nicaraguan authorities while in El Salvador and knew of "no other Nicaraguans or other foreigners" fighting alongside the guerrillas there.

Tardencillas also denied ever being in Cuba or Ethiopia. He said he gave that information to his Salvadoran captors initially as a delaying tactic to help his guerrilla comrades evade capture. "Because of those conditions, my own weakness and the torture, I agreed to go along with the publicity farce," he said.

Asked about his future, Tardencillas said he expected to be returned to El Salvador to face "a trial rigged by those powerful individuals there who have the courts under their control. I expect that I'll be sinking as a person. What else can I expect from those criminals and fascists?"

When reporters asked if he was now under Salvadoran or American control, Tardencillas replied, "As to whose control I am under, I do not know. As far as I am concerned, they are one and the same thing.

"I am aware of what is happening and very clear about what awaits me. I know whose hands I'm in. I'm a revolutionary and one of the risks is death. I am willing to accept it."

State Department officials said last night they did not know whether Tardencillas will be returned to El Salvador. He said that, since his arrival Tuesday, he had been confined in a hotel room that he estimated was a 40-minute drive from Washington.

Last night, the Nicaraguan embassy here delivered a formal note to the State Department saying its government holds the United States responsible for Tardencillos' safety and insists that he be released immediately to the custody of the Nicaraguan ambassador. It added that, in the meantime, Nicaragua expects that the prisoner will not be allowed to leave this country.