The man thought most likely to become Brazil's next president is maintaining a low profile now that his unofficial candidacy has become known.

"Do you want to do me a favor?" Gen. Joao Baptista de Oliveira Figueriedo asked a group of journalists recently. "Forget about me. Pretend that I had a heart attack and died."

While Figueiredo, 59, head of the national intelligence service, is trying to stay out of the limelight, a civilian candidate with widespread public support is campaigning like a North American politician.

Sen. Jose de Magalhaes Pinto, an important leader of the party that is expected to endorse Gen. Figueriredo, has announced his candidacy and has been criss-crossing the country in search of support.

A Gallup poll conducted in two major cities and published by a leading news magazine indicated that Sen. Magalhaes Pinto could easily defeat Gen. Figueriredo or either of the other two potential military candidates if a popular vote were to be held.

Brazilian presidents are not chosen by popular vote, however.

The standard procedure since the overthrow of the last civilian government in 1964 has been for the various military factions to agree on a candidate from within their ranks. He is then endorsed by ARENA, the government-sponsored political party, and approved by the ARENA-dominated Congress.

Although the term of President Ernesto Geisel, a general, does not expire until early 1979, presidential succession has become the subject of extensive maneuvering and speculation among military men and politicans here.

Figueiredo's lead in the informal jockeying for position became known in July when a former aide of Geisel announced his support for the intelligence director. Figueiredo, until then almost completely unknown, immediately became a national figure, with the press publishing detailed accounts of his background and several ARENA officials rushing to endorse him.

Within some military circles, however, Figueiredo, whose agency has the combined duties of the CIA and the FBI, is perceived as being too moderate. Within a matter of days, the name of another candidate, associated with the hard-line faction to be right of Geisel, was floated: Gen. Sylvio Frota, minister of the army.

Soon after that, Gen. Dilermando Gomes Monteiro, army commander in Sao Paulo, was suggested as a possible compromise candidate.

The situation was further complicated - and enlivened - early in August, when Senate President Magalhaes Pinto, of Minas Gerais, said that he would be a candidate. It marked the first time since indirect elections were instituted in 1965 that a civilian politician had run for the presidency.

Magalhes Pinto, 68, a banker whose presidential ambitions antedate the 1964 coup, has been one of the most influential civilian backers of the military government. As governor of Minas Gerais in 1964, he gave crucial support to the military men who were poltting to overthrow President Joao Goulart. In the late 1960, he was foreign minister in the government of Gen. Artur da Costa e Silva.

"I am a man of the revolution," he declared soon after announcing his candidacy, "and I support its goals." Nevertheless, his call for a return to civilian role is being seen by many military men as a serious threat to their dominance. Leaders of the Brazilian Democratic Movement, Brazil's only legal opposition party see it as a means for airing issues the military would prefer to see ignored.

The military, sensitive to event the most oblique criticism of its policies, has viewed this public discussion and speculation with a distinct lack of enthusiasm. When Figueirado's name was first mentioned, Geisel's office declared that it considered speculation over the presidential succession to be "inoportune and unpatriotic." Figueiredo has repeatedly refused to discuss the issue in public.

The Geisel government also has proved willing to take firmer steps to stop discussion of the succession.

When the Correio Braziliense, a daily paper in Brasilia, recently began a presidential poll of members of Congress it was informed that the government was "profoundly irritated" by its straw vote.

The paper subsequently told a Geisel aide that it would stop only if Geisel aide that it would stop the poll only if the government officially prohibited it. Finally, after two-thirds of the Congress had voted in the secret ballot, Minister of Justice Armando Falcao telephoned the paper's publisher and forced him to suspend the vote.

This hard line, however, has not prevented Sen. Magalhaes Pinto from campaigning, although the best he can hope for, says a political columnist with strong contacts among the military, is the vice presidency.

Last week, for example, in a statement that received front page coverage across the country, the canny senator said that if chosen as Geisel's successor he would be able to rule without emergency powers, which Geisel invoked earlier this year when he closed the Congress temporarily after a dispute over a judicial reform bill. He would be able to do this, Magalhues Pinto said, because of the popular support he would gain from the political and economic reforms he plans.

His fellow ARENA leaders are generally lining up behind military candidates, and the Democratic Movement leadership supports him only cautiously and reluctantly.