President Reagan leaves today with high hopes but modest policy expectations on a five-day trip aimed at promoting U.S. security, economic and political interests in Latin America.

The trip, which underscores a hemispheric interest that dates back to Reagan's presidential campaign, is also a back-door comment on how little the administration expects from the lame-duck session of Congress.

"We think there's more to be gained in South America," said one official succinctly.

Another official said the trip demonstrated Reagan's growing desire to play a more visible presidential foreign policy role. Even when the South American trip is counted, Reagan will have traveled far less abroad during the first two years of his presidency than his three immediate predecessors.

Now, with his working coalition in Congress damaged and perhaps shattered by the midterm elections, Reagan may be tempted to follow the example of other presidents who turned to foreign policy undertakings when they lacked the votes to push their programs through Congress.

Reagan is said to be far more comfortable with foreign policy issues now under the tutelage of Secretary of State George P. Shultz and national security adviser William P. Clark, a longtime intimate, than he was in the days when Alexander M. Haig Jr. was secretary of state.

The multi-purpose trip is being billed by the administration as one that will "promote democratic institutions in the hemisphere." But Reagan's scheduled meetings include a private talk with Guatemala's leader, Gen. Efrain Rios Montt, whom few would describe as a democrat of any sort.

This meeting, as well as separate talks with the leaders of Costa Rica, Honduras and El Salvador, is slated for Saturday. These talks are intended to underscore the administration's continuing determination to isolate the leftist government of Nicaragua, which Reagan and his aides see as a font of terrorism and unrest in Central America.

In his talks with Central American leaders, said one senior official last week, Reagan will be "making a reaffirmation of our support for those countries in Central America that have been threatened by insurgents."

The major focus of the trip, however, is not on Central America, but on Brazil, where Reagan will spend the next three nights in the remote national capital of Brasilia. He will also make a side trip to the Brazilian industrial center of Sao Paulo, the most populous city in South America.

Discussions with Brazilian President Joao Figueiredo are expected to focus on the troubled economy of Brazil, which one U.S. official described as "a superpower of the next century." The United States has objected to the Brazilian policy of subsidizing exports and Figueiredo complained in a speech to the United Nations that "the present economic policy of the great powers is destroying riches" and threatening a global economic collapse.

Reagan met Figueiredo in Washington last May and the two leaders got along well together. They are of the same generation (Reagan is 71, Figueiredo 64), and the U.S. president reportedly was impressed with his Brazilian counterpart's efforts to play a conciliatory role in the Falklands war.

Originally, Shultz had intended to make the visit alone to Brazil, but Reagan decided last month that the visit should be expanded to a presidential goodwill mission.

The Reagan administration recently has attempted to repair some of the damage which the Falklands conflict did to relations with Argentina and the rest of Latin America by supporting a resolution urging negotiations between Britain and Argentina over the disputed islands.

In Brazil, Reagan is expected to demonstrate that the United States intends to restore the undefined "special relationship" which then-Secretary of State Henry A. Kissinger proclaimed between the United States and Brazil during the Ford administration.

U.S. officials said that Reagan probably will soft-pedal his anti-Soviet rhetoric in Brazil because, as one of them put it, "Brazil simply isn't interested in East-West issues except as they affect trade and commerce."

From Brazil, Reagan will fly to Colombia for talks with President Belisario Betancur. The United States is interested in stopping the flow of illegal drugs from Colombia and also in Betancur's tentative peace overtures to leftist guerrillas in his country.

Then Reagan will go to Costa Rica, whose president, Luis Albert Monge, reportedly rejected a U.S. suggestion for a meeting of Central American leaders that would be held in the Costa Rican capital of San Jose. Monge wants the United States to focus on the economic needs of Costa Rica, the region's most democratic state.

While in San Jose, Reagan will meet with Alvaro Magana, the provisional president of El Salvador. He will then fly to Honduras where he will meet with President Roberto Suazo Cordova and the Guatemalan leader, Gen. Rios Montt.

Though Reagan will spend far more time with Monge than the other Central American leaders, the inclusion of the meetings with the El Salvadoran and Guatemalan leaders is likely to focus attention on insurgency problems in the region at the expense of Costa Rica's economic needs.