Anti-Sandinista leaders say they will intensify efforts to overthrow the Nicaraguan government if it does not respond by April 20 to what they contend is their last call for negotiations.

"We shall have reached the point of saying there is no solution with the Sandinistas," said Alfonso Robelo, head of the Revolutionary Democratic Alliance's political arm. Robelo, former presidential candidate Arturo Cruz and Alfonso Callejas, of the Nicaraguan Democratic Force, the largest guerrilla group operating in Nicaragua, are in Venezuela as part of a tour of Latin American nations to drum up support for a proposal signed March 1 by most opposition groups. It calls for negotiations among the Nicaraguan government, anti-Sandinista rebels and the civilian opposition. The opposition leaders conceded that there is little hope of a response from Nicaragua's Sandinista rulers, who barred Cruz from entering the country to deliver the proposal to church leaders earlier this month.

If, as expected, there is no response from the Sandinistas, the opposition will conclude that a solution to the country's civil war will only be obtained by force, Robelo said.

"Military options undoubtedly would grow stronger," said Cruz, who reiterated along with Callejas and Robelo that all Nicaraguan opposition leaders are opposed to a U.S. invasion of the country.

The proposal is supported by major opposition groups, with the exception of the military arm of the Revolutionary Democratic Alliance, headed by Eden Pastora. The plan rejects any solution to the fighting in Nicaragua imposed as a result of government-to-government negotiations that does not include the institutionalization of a democratic electoral system in Nicaragua.

"It's an error to attempt to bring peace to Central America through agreements between countries without taking into account what is going on inside the countries," Robelo said.

The proposal calls for a general amnesty, the withdrawal of all foreign troops and advisers from Nicaragua, and separation of the Sandinista party from the armed forces and other governmental institutions that it now dominates.

In exchange, the opposition groups would implement a unilateral cease-fire and would allow Nicaraguan President Daniel Ortega to remain in power until new elections are called.