Forecast - Brazil 2026 general elections: Chamber of Deputies

A predictive model is not one more poll. A poll interviews a sample and describes opinion at a given moment. A model interviews no one. It gathers what can be measured, published surveys, past election results and the electoral rules, and derives from them a distribution of possible outcomes, with their probability. It does not say a party will win so many seats. It says within which range it will finish, and what each candidate’s chances of election are.

Why it is sturdier than a single poll

Two pollsters published on the same day can differ by several points. The model aggregates every survey published in each state and registered with the Superior Electoral Court, taking into account their age, their size and the pollsters’ reliability.

Past polling errors

Polls get it wrong, sometimes badly. The model builds in the size of the errors seen in past elections, without assuming in advance that one side will be underestimated. That is what gives each candidate their chances, and each list its range.

From votes to seats

Brazil elects its deputies by proportional representation, state by state, under a complex allocation rule. The model starts from the last election’s results and the make-up of the 2026 lists, then applies the official rule. Tested on the 2022 votes, it recovers 1542 of the 1572 seats actually allocated.

Linked races

The October 4 elections are not independent. The vote for lists close to the two leading presidential candidates moves with the presidential vote, and the model takes this into account.

Twenty thousand simulations

A single forecast would give a false sense of certainty. The model therefore replays each race twenty thousand times, varying what is genuinely uncertain. The probability shown is the share of simulations in which the event occurs.

No probability at 0 or 100%

Uncertainty is this model’s product, never its flaw. The simulations are reproducible, and the forecast on the eve of the vote is frozen, then compared with the official results.

Sources

Election results and candidacies: Superior Electoral Court. 2026 polls: surveys published by pollsters and registered with the Superior Electoral Court.

Export chambre_2026_par_etat.csv ↗ · Export the forecasts as JSON ↗

Credits

All rights belong to Le Millénaire SAS, 47 rue de Vivienne, 75002 Paris.

Model designed by
Project director: William Thay
Deputy project director: Pierre Clairé
Political analysis and data: Matthieu Hocque
Data analysis, mathematics and validation: Florian Gérard-Mercier