President Carter's attempt to undermine a controversial Brazilian-West German agreement under which Brazil theoretically could learn how to make its own atomic bombs has become an issue of sharp national concern here.

Brazilians also feel betrayed because Vice President Mondale, on his first official foreign mission for Carter, took up the nuclear pact directly with West German Chancellor Helmut Schmidt without telling Brazil anything abaout it first.

Less than a year ago, when Henry A. Kissinger was Secretary of State, the United States signed a "memorandum of understanding" with Brazil, giving it special "consultant status" and pledging to confer regularly with the Brazilian government on matters of mutual importance.

The Mondale trip to Germany is front-page news here. The possibility that the United States could actually get West Germany to back out of the deal has rallied internal support for President Ernesto Geisel, head of Brazil's authoritarian military regime and a strong backer of the nuclear pact.

When Brazil and West Germany signed the nuclear agreement, in mid-1975, Geisel's office issued a statement saying: "All activities resulting from this pact will be destined exclusively for peaceful uses. We give assurances that fissionabale material will not be used for weapons or other nuclear explosive devices."

Brazil's sole legal opposition party has joined ranks behind Geisel and the official government party in defending the agreement. Rep. Joao Cunha, an opposition congressman from Sao Paulo state, said: "President Geisel, speaking in the name of all Brazil's 110 million citizens, should repudiate this U.S. maneuver."

Cunha claims that the United States is trying to keep Brazil as "an eternal supplier of raw materials" and wants to prevent this fast-developing South American nation from "sharing in the economic, cultural and scientific progress of the 20th century."

Diplomatic sources here say that in reality, if the United States succeeds in getting West Germany to renege on the nuclear agreement, there will be little Brazil can do about it. This may help explain why Brazil has suddenly begun a diplomatic strategy of talking about German honor and West Germany's reputation for keeping promises.

Brazil has repeated its justification for signing the nulclear pact so many times that most foreign correspondents here can recite the arguments by heart:

As a rapidly developing country, Brazil needs atomic energy to produce electricity for its burgeoning metropolitan areas and industrial complexes.

Brazil asked the United States in the past to supply it with equipment for enriching its own uranium, to guarantee a permanent supply of fuel for its nuclear reactors, but the United States refused. So Brazil turned to West Germany, which agreed to make available such know-how and equipment.

Brazil cannot run the risk of having its access to energy supplies controlled by outsiders, such as the United States - look what happened to the United States during the Arab oil boycott, Brazilians point out smugly.

Although the capability to enrich uranium, under the so-called full fuel cycle, will theoretically make it possible for Brazil to build atom bombs, Brazil is a peaceful country concerned with its own internal development than with foreign conquests.

The Brazilian-German nuclear agreement has been approved by the International Atomic Energy Agency, which sets rigorous standards against unauthorized uses of atomic power.

Eight atomic reactors are to be built in Brazil by 1990 under the agreement with West Germany.An official said that work under the agreement "is coming along on schedule, and in some areas you could say that we're moving ahead faster than we expected."

Construction has not begun on any of the German-designed reactors or the controversial uranium enrichment plant. Most of the work carried out under the agreement so far has been setting up and staffing the joint companies that are to build and install the nuclear equipment, along with some site-clearing and preliminary construction.

Brazil already has one nuclear power plant, which is expected to begin producing 630,000 kilowatts of electricity later this year. This plant, however, was built by an American company, and under U.S. law, the U.S. government will strictly control the amount of enriched uranium to be made available to it.