MEXICO CITY, DEC. 1 -- In response to a Sandinista cease-fire proposal made public in Washington last month, Nicaraguan rebels have called for a 40-day truce starting Dec. 8 as part of a "political negotiating process" that they said must lead to "irreversible democratization" in Nicaragua.

The two sides have agreed to meet as early as Thursday in the capital of the Dominican Republic, Santo Domingo, to begin indirect negotiations through a mediator, Nicaraguan Cardinal Miguel Obando y Bravo.

The two proposals appear to be so far apart, however, that any agreement on a cease-fire within a matter of days is considered highly unlikely.

The proposal of the Nicaraguan rebels, known as contras, was given last week to Obando, who turned it over to the Sandinista government in Managua yesterday after the Nicaraguan Bishops' Conference had formally approved his mediating role.

Marta Sacasa, a contra spokeswoman based in Miami, said in a telephone interview that members of a negotiating panel, formed in anticipation of cease-fire talks shortly after a Central American peace accord was signed in Guatemala Aug. 7, planned to gather Thursday in Santo Domingo.

The panel named by the contras includes Jaime Morales Carazo, a former banker in Nicaragua, two other members of the assembly of the Nicaraguan Resistance, the contra umbrella group, and two field commanders.

Nicaraguan President Daniel Ortega reportedly has named a negotiating team headed by Maj. Ricardo Wheelock, the intelligence chief of the Army.

Charging that the ruling Sandinista National Liberation Front has betrayed the revolt that ousted former dictator Anastasio Somoza, the contra proposal calls for the establishment of a democracy marked by political pluralism, civil liberties and respect for human rights.

The statement says the Nicaraguan Resistance has agreed to negotiate a nationwide cease-fire with the Sandinistas to last from Dec. 8 until Jan. 17, 1988. It says that during the cease-fire, rebel forces in Nicaragua would remain in their present "zones of operations," which the contras say cover 46 percent of Nicaragua's territory.

Both sides would be permitted to receive nonmilitary supplies by air, land or sea, the proposal says.

In conjunction with the truce, the proposal demands that the Sandinistas implement a series of sweeping political, military and economic changes.

Among the measures are a general amnesty for political prisoners, lifting of the five-year-old state of emergency, full freedoms for the press, political parties and unions, freedom of movement including the right to leave and enter the country, dismantling of "militarized cooperatives," an end to military conscription, dissolution of Sandinista neighborhood committees, discontinuation of rationing and a halt to "hyperinflation" and "administrative incompetence." Once democratization is achieved in compliance with the Guatemala peace accord, the proposal says, the "gradual disarmament of both parties" could begin, leading eventually to "a unified national army of a voluntary nature."

Cardinal Obando told reporters after delivering the contra proposal that he was sure each side initially would reject the other's conditions. He said he was empowered by both sides to make his own suggestions to achieve progress, and he said he hoped the indirect talks could lead to at least a temporary truce by Christmas.

During a visit to Washington last month, President Ortega unveiled an 11-point cease-fire proposal that called for the contras to congregate in three zones during a one-month truce to begin Dec. 5. During the cease-fire, the proposal said, military aid to the rebels would be cut off, but they could receive nonmilitary supplies through a "neutral institution."

There is a fundamental conflict between the rebels' demand for a broad political negotiation and the Sandinistas' insistence that the political future of the country is nonnegotiable.