IN MANY WAYS, Brazil's president, Luiz Inacio Lula da Silva, is a bright exception to politics as usual in Brazil -- and indeed in Latin America. Power in the region is too often the monopoly of well-born elites; Mr. da Silva rose from humble origins. Policies in the region too often veer between ruinous populism and punishing austerity; Mr. da Silva has combined macroeconomic stability with progressive social policies. Yet Mr. da Silva's administration is now mired in a corruption scandal. A member of his party was arrested this month trying to board a plane with $100,000 hidden in his underpants. His chief of staff and two other senior officials have resigned. His political future is uncertain.

Mr. da Silva's plight is part of a broader problem in Latin America. The region has successfully outgrown military coups, but political institutions remain weak and corruption is endemic. Since 1990 a score of Latin American presidents and ex-presidents have been charged with corruption, and their ignominious falls do not seem to deter more scandals. In 1992 another Brazilian president, Fernando Collor de Mello, was forced to resign after investigators discovered that his lieutenants had collected millions of dollars in bribes. "What may come out of all this is a turning point for reducing Brazil's pervasive corruption," Roberto Civita, a Brazilian magazine owner, told the New York Times then. If only.

Brazil's current scandal reflects the problem of institutional weakness as well as the tradition of corruption. Because of the weakness of political parties -- there have been more than 250 party switches by Brazilian lawmakers since 2003 -- Mr. da Silva has had difficulty putting together congressional coalitions to enact his legislative program. It looks as though the president's party used bribery as a short cut, though the president himself remains untainted. If this is what happened, it would not be the first time. In 2000 the administration of Argentine President Fernando de la Rua faced charges that it had bribed opposition senators to enact a new labor code. The vice president was forced out of office.

Most political systems feature factions that are held together by money. Latin America's weak parties exacerbate this problem -- there's no ideological glue, so financial glue does all the work -- and the region's presidential systems add further stresses. Legislatures have few incentives to support a president: If he is weak, lawmakers don't want to be associated with his program; if he is strong, lawmakers see no reason to build him up further. This creates a temptation to bully the legislature (thus Peru's President Alberto Fujimori suspended Congress in 1992) or, alternatively, to buy it. Presidents who virtuously resist both routes may end up looking impotent. Five years after his impressive presidential election victory, Mexico's Vicente Fox has failed to implement much of his program.

Some commentators, notably Arturo Valenzuela of Georgetown University, advocate a switch to parliamentary systems. But as Mr. Valenzuela concedes, there's little support for this option; in a referendum in 1993, Brazil's voters rejected it decisively. Latin Americans will probably live with separation-of-powers gridlock for years, and perhaps may eventually appreciate its charms. Despite the political scandal in Brazil, the economy is sailing along nicely.