Brazilian legislative seat distribution legal challenge

The Democratic Labour Party (PDT), alongside the Missão, PSDB, and Avante parties, has filed a petition with the Brazilian Supreme Federal Court (STF) to challenge the current method for distributing legislative seats. The legal action, identified as Direct Action of Unconstitutionality (ADI) 8.036 and rapporteur by Minister Gilmar Mendes, questions the “largest averages” rule and the “80/20” criteria used for electoral leftovers.

The PDT argues that the current formula violates political plurality by allowing large parties and federations to secure additional seats even when candidates from smaller parties receive higher vote counts. The parties are seeking an urgent injunction to implement changes before the January 2027 inauguration.

If the STF accepts the revision, the composition of the Chamber of Deputies and various state legislatures could change significantly. Projections suggest up to 36 seats in the Chamber of Deputies could change occupants, potentially replacing elected deputies with alternates. In Bahia, for example, projections indicate seven seats in the Legislative Assembly and six in the Chamber of Deputies could be affected. While parties such as MDB, PSOL, PSB, PSDB, PP, PSD, Republicanos, and PL could potentially benefit, parties including PT, União Brasil, Avante, PV, and PDT could see a reduction in seats.