San Jose, Costa Rica — International pressures are mounting on the Sandinista rulers of Nicaragua. One important new trend is that West European and Latin American leaders are becoming openly antagonistic to the Managua government. These same leaders once formed the backbone of democratic support for the Sandinista revolution.
This change is threatening to further isolate a regime already under military and economic attack from within and without.
''We are not going to keep defending a process that has become Marxist-Leninist. That is for some other group of nations or parties to do,'' says Daniel Oduber Quiros, a former Costa Rican president and vice-president of the Socialist International.
Mr. Oduber says this is the consensus of Socialist International leaders, including Willy Brandt of West Germany, SI's president, and the organization's two other vice-presidents, Spanish Prime Minister Felipe Gonzales and former Venezuelan President Carlos Andres Perez.
The SI leaders, says Oduber, articulated their concerns in a letter to the Sandinista leaders. That letter, so far unanswered, called on the Sandinistas to use the occasion of the July 19 anniversary of their revolution to announce a democratization of the Nicarguan government.
''We are more or less telling them this is the last effort we are making to defend them from other problems,'' Oduber says.
Those problems, he says, include mounting threats of military intervention against the four-year-old government. Oduber says he delivered the same message to Fidel Castro on a visit to Cuba last week. Oduber, speaking on behalf of the Socialist International, reports that Castro's response was terse. But he says the Cuban leader believes the Central American crisis can be defused through peaceful means.
Until recently the members of the Socialist International have been a valuable source of support for a revolution beleaguered by rebel attacks on its northern border and now on its southern frontier, and that has been treated as a pariah by the Reagan administration.
Socialist International backing for the Sandinistas has weakened gradually. But only in past weeks have the leaders taken what Oduber calls a more ''aggressive'' public posture. As recently as this spring the SI voted in favor of a resolution that, though it suggested a retreat from earlier enthusiasm, still said:
''We reiterate our support for the Nicaragua revolution because we support the democratic objectives of this revolution.''
It is not clear what tack the Sandinistas will take in the face of a continued erosion of international support. One source close to the Sandinista leadership suggests it may only drive the Sandinistas deeper into the Soviet sphere. But other observers suggest a new atmosphere of urgency - both in terms of peace initiatives and in war preparation - make the moment ripe for reform measures.
There is virtual consensus among observers here that unless a peaceful arrangement is reached quickly, regional war is a likely result.
''If no solutions are found within 30 or 60 days, I think armed conflict will increase,'' Oduber says.
Others offer different timetables. One Western diplomat gives the peace process three to six months. But no one seems to dispute that the region is moving to a war footing even as peace efforts intensify.
The best hope for a peaceful solution, observers here seem to agree, is the ''Contadora group,'' made up of Colombia, Mexico, Venezuela, and Panama. Although the group so far has produced little, it appeared to reactivate last weekend when the presidents of the four countries met in Mexico. It is seen as significant that Nicaraguan junta coordinator Daniel Ortega Saavedra, on the revolution's anniversary, backed the Contadora negotiation concept of multilateral talks. That position represents a sharp change for the Sandinistas, who previously insisted on bilateral negotiations with Honduras.
The announcement comes against a backdrop of increasing military pressure on the Sandinistas. The Democratic Revolutionary Alliance (ARDE) in the south and the US-financed Nicaraguan Democratic Force (FDN) in the north continue to battle the Sandinistas. The United States, which has cut Nicaraguan sugar imports by 90 percent, this week announced that eight naval ships would cruise the Pacific coast of Central America as a warning to Nicaragua. In addition, an aircraft carrier and battleship may be dispatched to the Caribbean as part of military maneuvers in neighboring Honduras. President Reagan said Thursday that the US maneuvers are not gunboat diplomacy and that he hopes the US will not find it necessary to blockade Nicaragua.
While such activity has raised regional tension, some suggest it also creates conditions conducive to concessions.
Hoping to take advantage of this atmosphere of pressure, a group of Nicaraguan dissidents plan to travel to Europe and Latin America next week, saying it seeks to reform the revolution through political means.
The group, which opposes the ''Sovietization'' of Nicaragua, is composed of Arturo Cruz Porras, once a member of the Sandinista junta and a former Nicaraguan ambassador to the US; Leonel Poveda, former Sandinista vice-minister of internal commerce; and Alfredo Cesar, former president of the Central Bank of Nicaragua.
''We (the Socialist International leaders) are backing their position fully. We want them to go ahead and see what they can do,'' Oduber says.
The Nicaraguan dissidents going to Europe have long been sympathetic to ARDE and to its military leader, Eden Pastora Gomez. But neither Mr. Pastora nor other rebel groups have been able to rally much international support.
In part, SI support for Mr. Cruz's group is seen as a reflection of the fact that Pastora, for reasons of personality, politics, and awkwardness in diplomacy , was an unacceptable representative of reform. But it also reflects SI's increasingly aggressive criticism of the Sandinistas.
Cruz argues that the Managua government can ill afford increased political isolation. Even Castro, reports Oduber, agrees. ''The Nicaraguan revolution needs friends not only among the communist countries but among the Western capitalist ones as well,'' Oduber says he was told by Castro.