Rebel negotiators will propose today that the Nicaraguan government join opposition parties inside the country in a power-sharing arrangement to prepare the country for a full return to democracy, two contra leaders said yesterday.

Adolfo Calero and Alfredo Cesar of the rebels' political directorate said this plan will be the centerpiece of their proposal on the bargaining table in face-to-face talks with Managua's Sandinista representatives in San Jose, Costa Rica.

To demonstrate that their goal is the restoration of Nicaraguan democracy rather than power for their movement, Calero and Cesar said that all contra political and military leaders would abstain from serving in the proposed interim government.

Instead, they said, all non-Sandinista members should be drawn from the 14 noncommunist political parties inside Nicaragua that collectively are known as "the internal civic opposition."

"We have no demands of our own," Calero said. "We are committed to support the internal opposition fully in their position that a cease-fire and democratization are essential for political reconcilation within Nicaragua and that a transitional government is the best way to achieve national unity."

Other aspects of the negotiating proposal call for any new lethal aid approved by the U.S. Congress to be put into escrow for 30 days, during which time the Sandinistas, contras and civic opposition must agree on a cease-fire to last a maximum of six months.

Nicaraguan President Daniel Ortega, under pressure from the presidents of Costa Rica, El Salvador, Honduras and Guatemala, agreed two weeks ago to direct negotiations with the contras and to take steps toward democratization in order to comply with the peace plan signed by the five Aug. 7.

However, the contras' power-sharing proposal seems to be far off the mark of what the Sandinistas might accept in the name of reconciliation.

In the past, Ortega has said only that Sandinista opponents, whether at home or in exile, can accept an amnesty and compete in future elections.

William Branigin of The Washington Post added from San Jose:

The head of a Sandinista negotiating team said U.S. congressional approval of any new aid for the Nicaraguan rebels would be "disastrous" for the Central American peace process.

In a news conference on the eve of the direct talks, Nicaraguan Vice Foreign Minister Victor Hugo Tinoco rejected the new $36 million aid formula put forward by the Reagan administration.

Tinoco said his delegation has brought new proposals but he declined to specify what they were.

Earlier, Costa Rican President Oscar Arias, the author of the Central American peace accord, said he feared that the reduced contra aid figure of $36 million had a better chance of approval than the $270 million originally proposed.

Arias has opposed new contra aid as inimical to the peace process.

In Madrid, visiting Nicaraguan President Daniel Ortega called on Spanish and other international leaders to verify the will for peace of his government and to urge an end to U.S. aid to the contras, special correspondent Tom Burns reported.