Secretary of State George Shultz, apparently stymied in his attempt to round up Central American support against Nicaragua, left Monday on a 12-day trip to Latin America to promote the administration's policy.

Shultz left Andrews Air Force Base in nearby Maryland for Guatemala City, where he is scheduled to meet with foreign ministers of Guatemala, Costa Rica, Honduras and El Salvador.This month's journey for Shultz had been set up as a continuation of his most recent trip to Central America a month ago, when he launched a sustained attack on the Sandinista leaders of Nicaragua. Shultz described the Sandinistas as threatening "odd men out" in a region moving generally toward democracy and economic prosperity.

But Shultz confirmed in a television appearance Sunday that there was no Central American consensus on a strongly worded communique denouncing the Sandinistas. Shultz said Central American countries could not agree on the wording of a joint communique, which he said was the work of the Central American foreign ministers themselves.

He did not comment directly on a report in The New York Times that said the impetus for the denunciation of the Sandinistas had come from the U.S. envoy in Central America, Morris Busby, and that the governments of Costa Rica and Guatemala had rejected such a strong denunciation.

In Managua, Nicaraguan President Daniel Ortega urged the four Central American countries to reject any U.S. attempt to condemn or isolate Nicaragua.

Ortega told reporters Sunday he hoped "the Central American governments will respond with dignity and firmness and reject the proposal of the United States."

After the one-day Guatemala foreign ministers meeting, Shultz heads to Argentina for his final tour of South America. He will also visit Brazil, Bolivia, Uruguay, Ecuador, Costa Rica, and El Salvador.

Meanwhile, Senate Democrats are working on a proposal to give military aid to the Nicaraguan Contra rebels if the Sandinista government does not honor the peace pledges it has made, according to a key architect of the plan.

President Reagan has already called for renewed assistance in the wake of a crackdown on the opposition in Managua.

The Washington Post said Monday that the Democrats' plan calls for a release of $18 million in frozen military aid to the Contras.