U.S.-sponsored mediation between Nicaraguan President Anastasio Somoza and his political opponents has reached a stalemate and appears to be fast disintegrating in the face of a Nov. 21 deadline the opposition has set for its withdrawal from the talks.

In consultation yesterday with William Bowdler, the U.S. representative on the three-nation mediating team who was temporarily called back to Washington Sunday, high-level State Department officials debated the advisability of applying increased U.S. and international pressure on one or both sides to keep the negotiating process from total collapse.

While both Somoza and the opposition have accused the United States of pressuring them, the Carter administration has repeatedly said it would not intervene in the Nicaraguan situation or press for a solution not agreed to by both.

Although sources said U.S. military intervention was not under consideration, the administration is now faced with the strong probability that the mediation will fail. The result likely would be renewed battles between Somoza's National Guard and the Sandinista guerrillas who led the opposition in a civil war last September.

Both the Guard and the Sandinistas are now believed to be much better armed than in the September battles. The Somoza government reportedly has received extensive weapons resupplies from Israel.

The U.S. government is now known to assume that the Sandinistas, which it believes are now receiving arms from Venezuela and Panama, have obtained surface-to-air missiles capable of shooting down National Guard aircraft.

The United States now believes that the only way to save the mediation effort is to persuade either Somoza or the opposition - a coalition of political, business and civic groups that backs the guerrilla actions - to modify demands before next Tuesday's deadline.

After six weeks of mediation, Somoza has rejected the opposition plan calling for his immediate resignation and the installation of a provisional government, and the opposition has rejected Sombza's counterproposal of a national plebiscite to determine popular support for each side.

Under the government proposal, handed to the mediators last week and presented in a news conference last Friday, Somoza would form a new government, in which the opposition would be represented according to the amount of plebiscite votes it receives, and he would remain as president.

The Board Opposition Front several days ago rejected the counterproposal and escalated its own demands to a call for Somoza and all members of his family to leave Nicaragua by Nov. 21.

Bowdler, and the other mediators from Guatemala and the Dominican Republic, who were called to their own countries for similar consultations, are expected to return to Managua for meetings with the opposition tomorrow.

While a final State Department decision has not yet been made on a course of action, sources said the United States is expected to propose - with the agreement of the other two mediating countries - a salvage compromise. It would involve a plebiscite, probably run by the Organization of American States, to determine the extent of popular support for Somoza.

Should he receive less than 50 percent of the vote, Somoza would then, under previous agreement, resign immediately and leave the country. A provisional government reflecting the vote ratio would be formed along the lines suggested by the opposition.

Among the pressures and inducements for acceptance of this plan now under U.S. consideration is the offer of asylum for Somoza in this country, along with an opposition agreement not to seek his extradition on criminal charges, should he lose in the plebiscite.

Should Somoza not agree, the U.S. range of pressure options include withdrawals of the U.S. military mission and possibly the U.S. ambassador to Managua, and the threat of public U.S. endorsement of the opposition proposal.