A SWEEPING victory for democracy was proclaimed last spring when, for the first time in its 170 years of independence, Nicaraguaconducted elections and the incumbents, losing, yielded power peacefully to the winners. But in the months since, Nicaragua's Sandinista minority has made grimly good on former Sandinista president Daniel Ortega's threat to ''rule from below'' -- to use its informal power in the streets and workplaces to thwart the formal power that the people voted to President Violeta Chamorro in the councils of government. This has put a miserable extra burden on the country's prospects for recovery from a decade of war, civil strife and economic duress.

While the Sandinistas ruled, their ''unions'' were pussycats, obediently controlling the workers they ostensibly represented and never striking. Only now, in opposition, have these organizations discovered a compelling class interest. In fact they represent only one faction of workers and only an anachronistic, discredited and repudiated set of political ideas. But they were able to immobilize Managua the other day in a violent political strike that wounded the city and the government and left at least five people dead. The strike ended with major government concessions that badly damage President Chamorro's economic recovery program.

Even among Mrs. Chamorro's Nicaraguan supporters, criticism can be heard of her patrician or dynastic political style. Her decision to keep on Sandinista Humberto Ortega as defense minister has drawn bitter fire, although so far it has politically neutralized the armed forces. The contras, now demobilized, complain bitterly of their difficult homecoming. Meanwhile, Mrs. Chamorro must labor under immense economic pressures as well as the evident political ones. Emory University scholar Robert Pastor notes that the Sandinistas left Nicaragua with a mere $3 million in reserves, a debt of some $11 billion and average wages that had declined 90 percent in a decade. American aid covers barely half of the foreign exchange requirements.

Governing in a country without a strong parliamentary tradition, President Chamorro has appealed for a ''national dialogue'' to make a new ''social pact.'' For this to work, she needs a cooperative patriotic response from the Sandinistas. The country's friends, in addition to offering more conventional sorts of aid through the government, must also find ways to apply their political influence to soften the ways of the opposition.