BRAZIL, BY REASON of its size and rapid growth, is probably the most influential example among the countries joining the world's industrial economy. They used to be called underdeveloped, but that term no longer applies. If you rank the world's nations by wealth and industrial sophistication, Brazil stands about two-thirds of the way up from the bottom and one-third of the way down from the top -- and over the past two decades, things have been moving much faster among the middle-income countries than among those at either the top or the bottom of the list. The political choices that Brazil is making are important not only for Brazilians but as examples to a world that constantly and unsentimentally assesses national patterns of advancement.

When the military seized control of Brazil in 1964, that constituted the most significant event in South America's long retreat from elected government. Similarly, Brazil's return to democracy is part of a great cycle running through all of South America. Currently only Chile among the larger countries is still ruled by a general. The inauguration of Brazil's newly elected president, Tancredo Neves, has been postponed a couple of weeks by his illness. But his vice president, Jose Sarney, has taken over the administration in a smooth and assured transfer of power.

This transfer does severe damage to a famiiar stereotype of South American politics -- the one that considers democracy to be a fair-weather form of government among South Americans, who, it alleges, will always turn authoritarian in the face of adversity. Brazil has been restoring democratic practice, in careful and measured stages, while it has been going through a drastic recession and an economic reorganization forced on it by the weight of its gigantic foreign debt. Rather than generating social upheaval and military repression, it has brought forward the adoption of a genuinely democratic government.

But it would be reckless to think that the present degree of hardship could continue forever without political effect, and here the United States has large responsibilities. The sudden increase in Brazil's debt burden over the past five years is the result of higher interest rates in the United States. It is possible to argue that Brazil's debts are tolerable even at these levels of interest, as long as it can sell its exports in a strong and rapdily expanding American market. But if the American economy should stop growing, or if Americans should try to close their markets to South American imports, the consequences for Brazil's people and their new government would be severe. The United States now has an obligation to support the great achievement of the Brazilians.