As part of a three-day visit to Washington, President Violeta Chamorro of Nicaragua addressed a joint session of Congress this week in a fervent appeal for more U.S. help for her struggling country. But the appeal has little chance of producing the help Chamorro seeks.

Chamorro was warmly welcomed in Congress. She has been something of a heroine ever since her upset of Daniel Ortega's Marxist-style government in February 1990 in that country's first free election under Sandinista rule. Ortega didn't think he would lose or he wouldn't have allowed the vote.Yet the election was not the end of Nicaragua's troubles. The Sandinistas have vowed to gain control once more. They still command the army, are an obstacle to economic reform and are still trying to export revolution. In addition, there have been a series of murders of former anti-Sandinista Contra rebels in recent months.

Despite sympathy for Chamorro and encouragement from U.S. officials, Nicaragua's cause has slipped almost out of sight. At Chamorro's speech before the joint session of Congress, only about 100 out of 535 members turned out for the event. That is smaller than usual for such gatherings.

There are several reasons for the loss of U.S. interest in Nicaragua and Central America in general.

First, the problems in the region were argued and fought over for much of the past two decades. There seems to be a certain weariness in Washington with the problem. Also, the rise of democracy in Latin America and the decline of the Soviet threat worldwide has taken some of the edge off the region as a kind of East-West battleground. The sense of crisis has faded.

Add to that the rise of the Middle East once again as a theater of urgent concern, complete with a shooting war involving the United States, and the shift of attention is understandable.

Second, the administration and Congress have their own money problems. Staggering budget deficits, the need for economic help in newly free nations in Eastern Europe, and the cost of the Persian Gulf war have all combined to limit the ability of the United States to respond to all the cries for aid around the globe.

Aid to Nicaragua was $335 million in fiscal 1990 but fell to $202 million this year. The administration is planning $179 million for fiscal 1992. Even those most sympathetic to Nicaragua's plight are doubtful there is anything in the foreign aid budget to allow more help. With a per capita income of $350 a year, Nicaragua is among the poorest countries in the hemisphere.

Third, there is a small but powerful conservative movement in Washington that is unhappy with Chamorro's rule, saying she is failing to move toward establishing a free-market society, has not privatized much of the business nationalized by the Sandinistas, is too close to the Sandinistas and has left investigation of murders of Contras in the hands of Sandinista-dominated police and security forces.

Yet there should be more understanding of Chamorro's position, which is hardly one of strength. She must try to build a democratic nation on the ruins of a civil war, on the ruins of a dictatorship, and in circumstances where the Sandinistas are waiting for her to fail so they can grab power again.

If she must also try to build on the ruins of economic collapse, the task may well be impossible. It would be far better for the United States to help Nicaragua now than to be faced with the same old pre-1990 trouble with Ortega back in control.