GUATEMALA CITY, FEB. 20 -- When cease-fire talks between the Nicaraguan government and the rebels were suddenly suspended yesterday by their mediator, Cardinal Miguel Obando y Bravo, each side immediately accused the other of negotiating in bad faith while seeking to influence the decisions of the U.S. Congress on military aid for the rebels.

But accounts by participants indicate that it was not pressure from Washington that broke up the negotiations, but rather longstanding mistrust among the Nicaraguans involved: the leftist Sandinista government, the cardinal and the rebels, called contras.

Obando's deep doubts about the Sandinistas' sincerity led him to call off hastily negotiations that, by admission of members of the cardinal's mediating team, could have continued if he had waited only a few hours.

The Sandinistas' suspicions that the cardinal is partial to the contras and that the contras are obeying instructions from the Reagan administration to avoid any cease-fire led the Sandinistas to fumble the significant diplomatic advantage they wielded as this week's round of cease-fire negotiations opened. That advantage came as a result of the U.S. House of Representatives vote Feb. 3 against new military aid for the contras.

The talks that began here Thursday were the second round of face-to-face cease-fire negotiations prompted by a six-month-old Central American peace process. The current round was to last three days, but the cardinal ended it abruptly at 2:30 p.m. yesterday, citing the government's reluctance to approve a five-point proposal he had offered unexpectedly Thursday night.

The contras are pressing for broad democratic reforms in Nicaragua before they will accept a cease-fire, while the government argues that those reforms are already under way through the regional peace process and wants only to work out the technical arrangements for a cease-fire. After the crisis yesterday, both sides now say they are ready to resume the negotiations as early as next week. But the cordiality of earlier rounds is shattered.

Obando's proposal, which he said was designed to speed up the talks, called for the government to free political prisoners, allow full freedom of expression, reopen a dialogue with the opposition parties and revise its draft law. He called for the contras to gather their guerrillas into cease-fire zones for a 30-day truce.

The Sandinista delegation immediately reacted negatively to the proposals, sources close to the Sandinistas said. They reportedly saw them as biased on the side of the contras. President Daniel Ortega, consulted in Managua by telephone, regarded them as an attempt by the cardinal to force the government to negotiate political, rather than technical, issues with the contras.

During a 30-minute meeting yesterday with the cardinal, "The government never indicated it would give a positive response. The government said it had decided not to take up the cardinal's proposal," said Msgr. Bosco Vivas, a member of Obando's mediation team.

The Sandinista delegation actually had a fallback position accepting Obando's plan, but the cardinal had already canceled the session by the time they were able to present it. Sandinista negotiators, wary of the cardinal, said they refused to give their acceptance earlier because they wanted to draw up a formal written document detailing exactly what they were accepting.

Ironically, the Sandinista delegation failed to see that the cardinal's proposal also dismayed the contras, who did not want to pull back their forces into special zones. "The Sandinistas refused to grasp that the cardinal gave them the best chance they've had so far in these talks," commented Azucena Ferrey, a leader of the Nicaraguan Resistance, the contra alliance.

During most of the Sandinistas' eight-year-old reign, Obando, Nicaragua's Roman Catholic primate, has been their most influential critic. Before the formulation of the Central American peace plan, Sandinista leaders frequently insulted him personally and harassed his clergy and followers. Yet Ortega asked Obando to mediate so as to lend credibility to the cease-fire talks.

Up to now, Obando has been restrained in his role, but he appears to have taken offense at the Sandinista maneuvers here. By suspending the talks, Obando thrust himself forcefully into the center of the negotiations, putting the government on notice that if he is to continue lending his prestige he wants rapid results and will not tolerate delays of even a few hours.