The political opposition to President Anastasio Somoza is trying to figure out a way to reject a U.S.-sponsored mediation proposal, designed to resolve the Nicaraguan crisis peacefully, without appearing to be intransignet.

The mediation proposal, presented to both sides Tuesday, call for a national plebicite in which Nicaraguans would choose between the continuation of the Somoza government or a "government of national unity" without him.

Somoza on Tuesday appeared to reject what he called the "hypoltheical" plebiscite proposal but the mediators obviously feel he has not given his final world on the question.

The U.S. plan, presented under the auspices of the Organizaiton of American States, is a last-ditch effort to avoid resumption of the civil war between Somoza's National Gaurd and civilians led by the Sandinista National Liberation Front guerillas.

Both Somoza and the Broad Opposition Fron coalition of 14 political, labor and civic groups are are expected to reply to the proposal by Saturday. The opposition had set last Tuesday as a deadline for Somoza's departure at the last moment, but that deadline has now been ignored by both sides.

While a few member groups of the Broad Front, primarily sectors of the traditional opposition Conservative Party and splinter groups of the government Liberal Party, are disposed to accept some form of the proposal, indications are that the majority of the Broad Front intends to reject it.

The mediators' plan calls for a plebiscite, to be organized and run by the OAS, within 60 days. The martial law now in effect would be lifted and there would be an amnesty for all political prisoners. Most significantly, the proposal calls for Somoza's prior agreement not only to resign but to leave the country with all members of his family should he lose.

One opposition alternative under discussion, but unlikely to be agreed on, would force Somoza to leave the country during the plebiscite itself, returning only if he wins more than 50 percent of the vote.

In some ways, the Broad Front backed itself into a corner by officially withdrawing on Tuesday from the mediation effort, run by representatives from the United States, Guatemala and the Dominican Republic.

The opposition argues that no Somoza agreement to resign and leave the country can be trusted and that any vote at this time would be invalid because of widespread popular fear of Somoza and the National Guard.

The key to the Front's opposition, however, is the knowledge that the Sandinista guerrillas, whose deadline to end a cease-fire was also Tuesday, are unlikely to honor any agreement that includes the possibility of Somoza's remaining in Nicaragua.

Because politics have been largely emasculated under the the Somoza government, few of the political groups of the Broad Front have a wide base of popular backing.

"The Saninistas," one opposition stategist said, "are the only group in the country with guaranteed widespread popular support."

What is unacceptable to the Sandinistas is likely to be unacceptable to the Nicaraguans who fought with them against the National Guard last September.

This week Somoza ended a curfew and some aspects fo broadcast ceasorship imposed in September and he has also alunched a large-scale public relations campaign in the United States.

The opposition fears that its rejection of what may appear as a reasonable, democratic proposal to the outside world could seriously affect international opinion.

Among the government's objections to the U.S. plan is a provision, based on an earlier Somoza referendum proposal that the opposition rejected, calling for a restructing of the government to give the opposition more representation even if Somoza wins a plebiscite.

As disscusions continued today, tension increased over the expectation that a guerrilla attack is inminent. While the guerrilla have made no large-scale move since their September retreat, the country seems to be in a constant state of anxiety.

The government distributed along the streets of Managua the transcript of a press conference held here last Saturday by Reps. Charles Wilson (D-Texas) and John Murphy (D-N.Y.). Both have been strong supporters of Somoza in the U.S. Congress. They travled here last week for talks with him and with U.S. Embassy and mediation officials.

During the news conference, both congressmen called for opposition. Acceptance of the government's original plebiscrite plan, rejected by the Broad Front tow weeks ago. That original proposal was similar to the one presented by the mediators Tuesday, with the major exception that it did not call for Somoza's resignation, no matter what the vote, but rather for a restructured government under his continued presidency.