Leaders of the three-year-old Sandinista government never had throngs of supporters in the United States. But now they don't even have Geraldine O'Leary Macias, as devoted a backer as the Nicaraguan revolution ever had.

She is an American -- 40, Minnesota-born, a former Maryknoll sister, a church social service worker among Nicaragua's poor since 1973--who worked in nonviolent ways to end the Somoza nightmare. She celebrated when the Sandinistas came to power.

Her Nicaraguan husband, Edgard, was a leader of the left-wing Popular Social Christian Party, which was politically aligned with the Sandinistas. A union organizer who survived the Somoza bloodiness, he served for a time as a deputy labor minister in the new government.

All that is finished. Edgard and Geraldine Macias and their two children recently left Nicaragua fearing for their safety. They are now at their parents' home in Minnesota.

The reasons for the Macias' self-imposed exile are part of an unfolding story that suggests the Sandinista circle has its own destructive excesses against human rights and civil liberties. Edgard and Geraldine Macias are only two of several of the revolution's true believers who are now out of the country and convinced that the people's victory of three years ago may be slowly turning into a defeat of the promised reforms.

Troubles for the couple began in June with the publication in the Soberania magazine of an article titled "In Nicaragua: the CIA Finances the 'Opposition.'" The story alleged that "the CIA has not only offered support to reactionary political parties, but also a donation of $250,000 arrived to Edgard Macias and another lesser sum to Geraldine Macias for 'diversionist' work. . . ."

While in Washington a few days ago, Mrs. Macias said that at first she and her husband thought they were the objects of a vendetta by the magazine's editor, who had political differences with them. But when they filed a lawsuit against the editor for slander, the news story about the suit in La Prensa, the Managua daily, was censored by the government. The attacks were also repeated on government-controlled television and radio.

Geraldine Macias doesn't have the full story on what she calls a smear campaign. She tells of a friend of her husband within the security police who warned the couple that the police had labeled Edgard "a dangerous person who should be eliminated" and Geraldine as "an undercover CIA agent."

Since returning to the United States, Mrs. Macias has spoken to church and political groups who supported the Sandinistas in 1979 and back it now. She reports that some listeners can't bring themselves to believe that her mistreatment means anything beyond an isolated incident. None, she wrote in a letter to friends, "wished to hear that another bloody confrontation is a real, tangible possibility: not because of the revolutionary process but because of the consolidation of power and privileges in the hands of a reduced clique that has isolated itself from the people."

Asked about the Macias case, an official at the Nicaraguan Embassy in Washington wondered what all the fuss was about. The couple, he said, could have stayed or left. It was their choice. He had no comment about alleged CIA connections.

That won't do. If the government cared about not only its reforms but its image among its friends in the United States it would have rallied to support the Maciases. The unfinished revolution needs this couple's talents and ideas, even if they are not in full alignment with the Sandinista methods of reform.

The government has ample reason to feel under pressure. The Reagan administration, following the 1980 Republican Party platform that denounced the "Marxist Sandinista takeover of Nicaragua," has been a shameless harasser of the Nicaraguan government. It should have been helping with economic aid. In southern Florida, right- wing Somoza exiles have been receiving military training in hope of returning their country to the pre-1979 terror.

In general, the revolution has led to some genuine reforms. But none of this justifies turning on a man and woman who have worked for a free Nicaragua as energetically as any of the country's revolutionaries.