The United States, in a new effort to obtain a moderate political solution to Nicaragua's escalating civil war, yesterday called for an urgent foreign-minister-level meeting of the Organization of American States.

In response to the U.S. request, the OAS tentatively scheduled the meeting for 3 p.m. Wednesday. Although the meeting technically will be of foreign ministers, most of the hemispheric body's 27 member countries are expected to be represented by their regular OAS ambassadors.

Underlying the U.S. move is the growing conviction within the Carter administration that urgent action is required, both to induce President Anastasio Somoza, Nicaragua's long-time dictator, to surrender power and to prevent an eventual takeover by radical guerrillas of the leftist Sandinista National Liberation Front.

Since an earlier outbreak of civil was last September, U.S. officials have made no secret of their belief that Somoza should step aside in favor of Nicaragua's more moderate opposition forces. That position was reiterated yesterday in a State Department statement that Nicaragua's problems should be resolved by the "establishment of a broad-based representation government."

In pursuing that goal, however, the administration's freedom of action has been hampered by President Carter's pledges not to intervene in the affairs of other countries.

For that reason, the United States is anxious to cloak any new initiatives toward Nicaragua in the respectability of a multinational undertaking that has the sanction of a regional group such as the OAS.

U.S. officials said yesterday they still were uncertain of what course, if any, the OAS might take. The United States, they added, plans to use the time until Wednesday for intensive consultations to try to determine if a plan acceptable to the majority of OAS members can be found.

Wednesday's meeting technically will be a continuation of one convened last September to deal with the Nicaragua situation. Then, divisions between the region's dictatorships and democracies, and traditional Latin American hostility to intervention, only permitted agreement on a limited and ultimately unsuccessful mediation effort.

U.S. officials said that since then, fears that the Nicaragua violence could spread and involve other countries may have caused some OAS members to reassess their positions.

These officials noted that Mexico, long an unswerving champion of non-intervention, now has broken relations with Nicaragua and become more outspokenly hostile toward Somoza.

There also are increasing signs, the officials added, that South America's Southern Cone military regimes-Brazil, Argentina, Chile, Uruguay and Paraguay-are starting to regard Somoza as an embarrassment. A few days ago, Brazil publicly cited Nicaragua as a "threat to the peace of the region" and called for a solution that would take into account "the legitimate interest" of all the parties there.

But, these officials cautioned, it is still too early to tell whether these changing attitudes will produce a majority in favor of action to put new pressures on Somoza and the Sandinistas to accept a moderate compromise.

Another reason for seeking concerted OAS action, some U.S. officials said privately, is to ward off further unilateral steps such as that taken Sunday by the five nations of the Andean Pact. The five recognized the Sandinistas as a "legitimate army"-a step that came close to acknowledging them as the legitimate power in Nicaragua.

Although the administration wants Somoza out, it is skittish about a Sandinista victory because the guerrilla movement contains at least some forces that are avowedly Marxist and pro-Cuban. As a result, the State Department reacted to the Andean Pact's move yesterday with coolness.

The department was more upbeat about the Sandinista announcement Sunday of a provisional government including members from its own ranks and from more moderate opposition forces within Nicaragua.

A department spokeswoman, Jill Schuker, said: "The political forces represented in the announced provisional government clearly would play a role in developing the eventual political solution."

In a full-page advertisement in yesterday's New York Times, U.S. supporters of Somoza reprinted letters sent to Carter by 100 members of the House and five senators. They called on the president to support Somoza as a traditional ally and warned that his overthrow would lead to a Marxist-controlled regime in Nicaragua.

On the other side, Sen. Edward Zorinsky (D-Neb.), chairman of the Senate hemispheric affairs subcommittee, yesterday sent Carter a letter urging that the United States take whatever political or military steps are necessary in conjunction with the OAS to force Somoza out.