In making its case against the Sandinistas in Nicaragua for complicity in terrorist acts in El Salvador, the Reagan administration does not hesitate to offer in evidence sketchy testimony from Napoleon Romero Garcia, a captured Salvadoran rebel leader who turned state's evidence.

But when it comes to an unfriendly witness with far better credentials . . . well, take the case of Edgar Chamorro. He's an unswerving supporter of the purposes of U.S. policy who happens to think the administration is going about the business of promoting democracy in Nicaragua all wrong. So the administration would be obliged if Chamorro would abandon his exile in Florida and get lost.

Chamorro, you see, has met the enemy of democracy in his native land three times. His family led the political opposition to the Somoza tyranny. Next, he watched the once promising Sandinista revolution succumb to Cuban and communist influence just as Ronald Reagan says. Then he linked up with the counterrevolutionary (contra) movement promoted and manipulated by the CIA and recently restored to good standing in Congress with $23 million in "humanitarian aid."

For more than a year, he was the CIA's handpicked candidate of the seven-member directorate of the Nicaraguan Democratic Force (NDF), the political arm of the contra fighting forces. That's when he met yet a third enemy of democracy in Nicaragua -- and it is us.

Or so he alleges in "Confessions of a Contra" in a recent issue of The New Republic. In the course of richly documenting the case, Chamorro suggests a larger lesson about the way the United States, and not just the Reagan crowd, behaves in these matters: arriving late with a heavy hand and a tendency to lose sight of democratic principles when their sanctity gets in the way of commie-bashing.

Chamorro and some like-minded Nicaraguan exiles began organizing in 1980, he recalls. They saw the Sandinistas growing "more repressive (and) became convinced that they had to be replaced and that only armed opposition could do it." At that point, he was on Reagan's wavelength as a shining example of right- thinking Nicaraguans who were, as the president put it, "denied participation" in the Sandinista government.

But in less than a year, Chamorro concluded that he was not at all on the CIA's wavelength. As he came to see it, the agency was exploiting him: timing the activities of the NDF to fit the U.S. congressional agenda, showing indifference to contra atrocities, coming up short with promised weaponry and insisting on making the important decisions.

After giving Chamorro and his associates a promise in November 1982 that they would be victorious by July 1983, the CIA was giving the impression by 1984 that it "didn't want to let us win."

Chamorro went public with his disillusion on American op-ed pages and in congressional corridors. In late 1984, he was relieved of his duty by the NDF directorate. His summing up what he had learned begins with his belief that "a political dialogue should be the United States' top priority. We have tried military pressure and it hasn't worked. . . . The first step toward national reconciliation should be abolition of the contra army. . . . President Reagan should also lower his inflammatory rhetoric and give more than lip service to the Contadora peace initiative."

He still thinks there are idealistic young people with valid grievances fighting against the Sandinistas. "But they are being used as instruments of U.S. foreign policy by the CIA and the Reagan administration . . . and by the old Somozista gang to get back the money and the power they lost in 1979." His crushing summation: "If the contras ever took power, they would simply replace the communists with their law-and-order regime and no one would be any better off. . . . I am now convinced that the contra cause for which I gave up two years of my life offers Nicaragua nothing but a return to the past."

Small wonder that the administration does not wish to put on the witness stand a man who sees no happier prospect at the hands of those whom Ronald Reagan sees as "freedom fighters" than he does at the hands of the Sandinistas.