Two Nicaraguan opposition leaders who traveled to Washington to talk with President Carter have been told that he is too busy to see them, but that the United States will carefully consider a communique, they delivered from the Sandinista Liberation Front guerrillas.

The communique says that the leftist guerrillas give their "total backing and approval" to a provisional government and eventual democratic elections proposed by the opposition to President Anastasio Somoza.

The document was delivered by Carlos Tunnermann and Ramiro Cardenal, representatives of the Broad Opposition Front coalition of political, business and civic groups who have joined the guerrillas to oust Somoza.

Concern over the possibility that the Sandinistas, who at various times in their 16-year history have espoused Marxist ideology, would take over in Nicaragua has in large part prevented the United States from taking a firm anti-Somoza position in that country's current political crisis.

At the same time, the administration's attempts to cut traditional U.S. ties to the opressive Somoza dynasty have been restricted by a policy of nonintervention. That policy, informed sources said, makes it difficult for Carter to talk directly to members of the Nicaraguan opposition.

The United States is currently engaged in an effort to bring both sides of the conflict to the negotiating table with an international team of mediators.

That effort has been delayed for nearly a week by disagreement between Somoza and the opposition over which countries - Latin American military governments or democracies - will serve as mediators.

A tentative group including the military, pro-Somoza governments of Guatemala and El Salvador, along with the United States, Colombia and the Dominican Republic, has apparently been agreed to by both sides.

Colombia may be replaced, however, perhaps with Mexico, following a strongly worded anti-Somoza letter sent jointly to the United Nations two days ago by Colombian President Julio Cesar Turbay and Venezuelan President Carlos Andres Perez.

Perez, who has been one of Somoza's loudest critics, also wrote a personal letter last week to Carter telling him that the Nicaraguan situation "is a dramatic danger to your human rights policy." U.S. actions so far, Perez said, "do not permit me to be otpimistic about the U.S. attitude toward the bloody Somoza regime."

Informed sources said from Managua yesterday that negotiating sessions between Somoza and the coalition Broad Opposition Front may begin as early as the end of next week.

Although Somoza has insisted, and the United States has apprently promised, that there will be no prior conditions to the talks, Tunnermann and Cardenal said yesterday that in their conversation with U.S. officials here they have insisted that the opposition will not agree to any negotiated settlement that includes Somoza's continuation in power.

The Sandinista communique delivered yesterday repeated that demand. The guerrilla prescription for an "effective and permanent peace" - and the conditions under which the Sandinistas will lay down their arms - include Somoza's resignation from the presidency, and the resignation of Somoza and all members of his family from the Nicaraguan National Guard.