HAS PRESIDENT CARTER, by barging in hard, toughened rather than softened Brazil's determination to build a nuclear-power system with a potential for later producing a bomb? This is at least suggested, though not proven, by the hotly nationalistic Brazilian reaction to Mr. Carter's initial handling of the issue. Before his election Mr. Carter pronounced Brazil's nuclear program unacceptable. Once in office, he dispatched Vice President Mondale to get West Germany to revoke its treaty commitment to sell Brazil a full nuclear-power cycle including technology usable for bomb-building. He did so, moreover, without any real consultation with Brazil.

The response from Rio was about what you'd expect. Even Brazilians skeptical about their country's nuclear program on economic grounds were so aroused by what they took to be Mr. Carter's insensitive bludgeoning that they swung to support the government's full-cycle position. Brazil, which has yet to accept the nonproliferation treaty, insists nonetheless that it won't build its own bomb. But it has so far refused to accept formal international constraints. Mr. Carter's attack got its pride up.

Quiet diplomacy is being tried. Deputy Secretary of State Warren Christopher popped into Rio the other day to offer 1) the respectful consultation due a power of Brazil's importance and ambition, and 2) proposals meant to support the Brazilian nuclear program even while removing from it the enrichment and reprocessing technology that lends itself to proliferaton. Discussion on these proposals will continue. Therein lies the hope.

One key is to assure Brazil a reliable fuel supply. That is how to deflate its claim to need its own enrichment and reprocessing technology. But there's a hitch. Brazil wants American uranium for its future reactor, which Westinghouse is about to turn over. But for the eight-plus reactors it plans to buy initially from Germany, its seeks uranium from a European combine.The answer lies in the sort of international cooperation that is extremely hard to pull together on the fly.

Brazil's reluctance to seen bowing to American dictation of its nuclear policy is understandable. But, realistically speaking, that is not a possibility. Brazil is not South Korea, a small dependency which, under American pressure, abandoned similar plans. Brazil is a large independent nation. It cannot be pressured; we hope President Carter accepts that now. It can perhaps be convinced that a prime world leadership role is open to the country which best demonstrates how to combine nuclear-energy and nuclear-security requirements. The United States would surely improve it chances of so convincing Brazil if it set its own example, in its arm control policies, of wise self-restraint.