Next few weeks will be crucial for those with stake in Nicaragua. All sides expected to maneuver for advantage as deadlines near

In the battle over Nicaragua, all the sides are keeping the powder dry. This week saw developments in Congress, at the White House, the United Nations, and in Nicaragua itself that could affect the future of that battle-wracked Central American country.

But the real testing time for American policy in Central America comes in the next few weeks. In the meantime, it is a time of maneuvering for advantage for all with a stake in the country and region.

This week, Costa Rican President Oscar Arias S'anchez addressed Congress and the UN and met with President Reagan, advocating a peace plan for Central America that has won the support of five Central American countries, including the Sandinista government of Nicaragua. The plan quickly eclipsed the Reagan administration idea for bringing an end to the Nicaraguan conflict.

It also threw into question the administration's continued insistence on US military support for the contra rebels seeking to overthrow Nicaragua's Sandinista government. This week, Congress approved $3.5 million in humanitarian aid for the contras, but gave no further military aid.

That is not the last word, however. The appropriation was part of a stopgap measure designed to pay the government's bills for the next 41 days. Congress will be forced to confront the contra aid issue when it approves a regular budget later.

By the time the budget is hammered out, partisans on both sides of the contra aid issue will have additional support for their arguments.

By the end of September, the administration's peace initiative for Central America will have run its course - so far, without notable results. And by Nov. 7, the peace plan supported by Mr. Arias and the other Central American nations should be taking effect. It calls for a negotiated cease-fire, political liberalization within Nicaragua, and an end to US military support of the contras.

The Sandinista government is taking some moves to support the Central American initiative, but they amount to a decidedly mixed bag.

The Sandinistas have, for example, allowed the opposition newspaper La Prensa to resume publication, but under strictures that would keep it from reporting on a number of controversial topics. Radio Catolica, the Roman Catholic Church's radio station, will also be allowed to reopen. Some political prisoners have been released, and some exiles have been allowed to return to Nicaragua.

The peace plan also calls for a negotiated cease-fire. But instead of entering into discussions about how to bring one about, the Sandinistas declared a unilateral - and largely unsupervised - cease-fire, without assenting to independent monitoring.

These moves by the Nicaraguan government have given hope to some and ammunition to others.

Elliott Abrams, an assistant secretary of state, denounced the Sandinista cease-fire as ``a trick,'' an effort to force the contra rebels into laying down their weapons while the Sandinista forces regrouped and resupplied for renewed fighting.

Even Arias, the principal backer of the Central American plan, said the unilateral cease-fire was not enough. ``Both parties will have to agree'' to the terms, he said.

A negotiated cease-fire, however, would force the Sandinistas to negotiate with the contras - something it has so far steadfastly refused to do.

Sandinista opponents in Nicaragua criticized the other reforms that have been carried out by the government, saying they were tentative and arbitrary.

But for many in Congress, the verdict is still out. US House Speaker Jim Wright predicted that the humanitarian aid voted this week to the contras will probably be the last American funds allocated to the contra cause. He declared himself ``optimistic'' about the chances for peace in the region.

A congressional staff member says that practical politics dictate that Congress, for the time being, refuses to vote more military aid. ``Everyone's got to give this thing a chance,'' he says.