MANAGUA, NICARAGUA, JAN. 7 -- A consensus is emerging among the Central American nations for extending the deadline for compliance with a regional peace plan beyond the Jan. 15 meeting of the five participating presidents, according to diplomats and governmental officials.

In Nicaragua, for example, the political opposition and the main monitors of the plan's progress have concluded that the leftist Sandinista government failed to comply fully with the democratic changes required under the Aug. 7 accord. But they say they want to see the peace process continue.

A previous deadline, Nov. 5, proved too early for full implementation of the plan. The probability that the pact's second deadline will be missed heightens the dilemma of Washington legislators awaiting clear signs from Central America to decide which way to vote on military aid for the Nicaraguan rebels, known as contras. That vote could come in February.

Many diplomats say the outcome of the first Central American summit since the peace accord was signed will be determined largely by the interaction of the five leaders during their day-long encounter, which is to take place in San Jose, Costa Rica. There could be surprises.

The governments of Costa Rica, Nicaragua and El Salvador have expressed interest in extending the deadline for a short period and have specified areas requiring further action.

"No one wants to take the historical responsibility for killing this peace process," observed a Central American diplomat.

Many diplomats expect new pressure to be brought on Nicaragua for a wider democratic opening, and they anticipate that Costa Rican President and 1987 Nobel peace laureate Oscar Arias will play a key role in this. He is likely to use the prestige of his prize and the threat from Washington of renewed contra aid to press for new political concessions from the Sandinistas, diplomats said.

"He will be wielding the sword of Damocles," said a Central American ambassador.

The Sandinista government forfeited much of the momentum initially gathered in Washington when, last month, Nicaraguan Defense Minister Humberto Ortega confirmed plans for a Soviet-supported military build-up eventually to arm up to 600,000 Nicaraguans. The speech had less impact at home. But Congress subsequently voted $13.5 million in interim contra aid. President Daniel Ortega has taken no major new peace-plan initiatives since then.

"The atmosphere has changed. Ortega's room to manuever is definitely reduced," said a western ambassador.

The government's earlier measures, allowing some opposition media to reopen, entering into political dialogue with the unarmed opposition and starting indirect cease-fire talks with the contras, have sparked heated debates in the Sandinista ruling party and the military.

Cardinal Miguel Obando y Bravo, the mediator with the contras, gave an international monitoring commission that visited here this week more guarded criticism of Managua's performance than many opposition leaders expected. Also head of a national commission charged with assessing the progress of the peace plan, Obando is the key arbiter in Nicaragua of the government's compliance.

Obando limited himself to saying, "We recognize the positive steps the government took," then cited the steps he felt the government failed to take, a much longer list. While the government pardoned 985 prisoners in November, he stressed the need for a general amnesty.

Asked about renewed U.S. aid for the contras, explained by Washington as a means of pressing for compliance with the plan by Managua, Obando said yesterday: "To the extent our government makes democratic reforms, any idea of that money would remain frozen for all eternity."

Informally, Obando has been sharply critical of both the Sandinistas and the contras for the impasse in the cease-fire talks since late December, diplomats and Roman Catholic sources said. He has accused the contras of breaking an agreement they made with him and of balking at the last round of talks in the Dominican Republic. He has suggested that neither side has more than short-term tactical reasons for entering into the talks.

But Obando has given no indication of giving up on the peace process.

Nicaragua's 14 opposition parties, in their report to the international commission this week and in an unusual display of unity, agreed that the war and political crisis had "deepened," since the peace pact was signed.

The parties called for an end to the state of emergency and broadening of the rights guaranteed in the constitution. But they reiterated their support for going on with the peace plan.

In an interview today, Nicaraguan Foreign Minister Miguel d'Escoto rejected reported accusations by El Salvador in closed sessions with the verification commission that Nicaragua was still aiding the Salvadoran rebels.

"We are ready for the commission to go anywhere in Nicaragua without any advance notice to inspect whether those charges are true," d'Escoto said.