The Petrobras scandal currently engulfing Brazil is unprecedented in its scale and scope. Ironically, when the party of President Dilma Rousseff, the Workers’ Party (PT), initially became a major political player in 1989, it was seen as a clean, ethical alternative after President Collor de Mello stepped down from office amidst corruption allegations. Yet in the years following its rise to power, the PT has been dogged by corruption allegations, even before the explosion of the Petrobras investigations. During the presidency of Ms. Rousseff’s predecessor, Lula Inácio da Silva, prosecutors unearthed a major scheme, known as the Mensalão scandal, under which public funds were being used to pay members of Congress in exchange for their support of the PT government in crucial votes. At the end of the inquiry, 25 politicians and businessmen were convicted. Several other smaller corruption schemes (including Caixa Dois, Bingos, Sanguessugas, and Dossier) also implicated high-ranking members of the PT during Mr. da Silva’s tenure.

Despite this clear evidence of corruption within the PT ranks, the PT has been able to maintain its relative dominance in Brazilian politics, with three successive victories in presidential elections following Mr. da Silva’s initial rise to power, including Ms. Rousseff’s re-election in 2014, six months after the launch of the Petrobras investigations. This raises yet again a question that scholars and commentators have asked over and over again: Why do voters keep re-electing corrupt politicians? Democracy is supposed to enable voters to hold their government accountable, and most voters claim to dislike corruption and to value integrity in government. So why do parties like the PT keep winning elections? While there are many possible explanations (maybe, for example, voters don’t really care as much about corruption as they claim), part of the explanation in certain countries may have to do with the particularities of the electoral system.

Brazil has a hybrid electoral system: the President is elected in a two-round majority run-off system, elections for the Senate are based on plurality votes within states, and elections to the Chamber of Deputies are based on open-list proportional representation. An examination of this system suggests that it is particularly inimical to holding corrupt politicians accountable, and may have in fact contributed to the seemingly intractable problem of corruption in Brazilian politics. Three problems in particular stand out:

• First, the open-list proportional representation system in Brazil’s Chamber of Deputies means that a large number of parties garner a small amount of seats; as a result, a governing party must form a coalition with a range of partners who are often dissimilar on ideological grounds. For example, three of the parties that supported the PT coalition in Congress under Mr. da Silva’s first term were ideologically distant from the PT; to secure their support, rather than give their leaders cabinet assignments, the PT opted to give them control over certain state resources and paid them for their support. While corruption is not inevitable in such a system, the need to maintain coalitions of ideologically diverse partners certainly encourages corrupt backroom dealing. According to Marcos Valério, the Brazilian businessman at the center of the Mensalão scandal, the PT spent US$40 Million a year to secure congressional support.
• Second, the legislative fragmentation produced by Brazil’s electoral system also tends to undermine incentives for inter-party monitoring and whistleblowing. We often imagine that opposition parties will be quick (perhaps too quick) to unearth and publicize evidence that politicians from rival parties have engaged in malfeasance. But when there are many small parties in Congress, politicians may be reluctant to criticize or monitor members of other parties who might be future coalition partners. For example, the PMDB, the party Michel Temer, who recently replaced Ms. Rousseff as the interim President following her impeachment, has formed coalitions with every governing party for the last two decades. Given the shifting nature of coalitions, true “opposition” to the governing party may not meaningfully exist. The current scandal in Brazil has seemingly implicated the entire political class: the politician leading the charge for President Rousseff’s impeachment, Eduardo Cunha, is himself being investigated in association with the Petrobras scandal.
• Third, Brazil’s system allows Presidential candidates and party leaders to insulate themselves from their parties, weakening the accountability mechanism. The President is elected through a majority run-off system, which produces a very personality-based electoral campaign, while the open-list proportional representation system for electing the lower house of Congress tends to produce more candidate-centric elections than would a closed-list system. Individual candidates to try to win a “personal vote,” rather than a vote for the party, and can often foist blame for past corruption scandals on the party. In 2006, Brazil’s electorate considered the PT to be the most corrupt political party, but Mr. da Silva was still decisively reelected, carrying a similar percentage of the general vote to his election in 2002. Similarly, Ms. Rousseff was able to insulate herself from her party’s corruption and was reelected for a second term in 2014. Though the PT was tainted by the Mensalão Scandal, and the Petrobras investigations had been launched, Ms. Rousseff actually had a reputation for being tough on corruption, having fired seven Cabinet members in a period of 15 months during her first term on evidence of their corrupt activity. Ms. Rousseff’s tough-on-corruption strategy shifted the reputational costs of corruption towards the PT and other coalition parties, contributing to her re-election.

Certainly, in designing a political system, there are other considerations in addition to the accountability mechanism of democracy in reducing corruption. But in addressing the root causes of corruption, structural reforms as deep as the very nature of the political system itself ought not to be immune from consideration.