Brazil's 2026 presidential election: the probabilities

1 The bottom line

• The favourite is Flávio Bolsonaro. Chance of becoming president: Flávio, 71% to 80%; Lula, 20% to 29%. The top end for Lula is our main estimate; the bottom end is the same estimate without one softening assumption about the runoff (see “What the model does not know”). We give a range rather than a single number because the honest answer depends on that assumption.
• The favourite is not the candidate leading the polls. In the first-round poll average Lula is on 44.3% of valid votes and Flávio on 42.1%. But in all 6 past elections, 2002–2022, the Workers' Party (PT, Lula's party) did worse against its rivals than the polls a few days before the vote suggested (table and sources under “How we calculated”). The model carries this lesson over to 2026, so Flávio's chance of finishing first in the first round is 64% and Lula's is 36%.
• The chance of a runoff is 74%. In head-to-head polls of Lula against Flávio, Lula is on 49.4% of the two-candidate vote. Each one-point shift in Lula's share in these polls moves his chance of winning by about 4.8 points: one point more for Lula takes it to 34%, one point less to 24%.

The key assumption in one line. In 4 of 5 past cases, runoff polls taken before the first round overstated PT; we carry this correction over in softened form (on average 2.3 points against Lula). If this time the head-to-head polls do not systematically overstate Lula, his chance is 41%.

2 Who finishes first in the first round — and why we part ways with the polls

Lula leads the poll average. The model takes into account that in all 6 past elections PT's first-round gap to its rivals came out worse than in the polls, and shifts the gap by 4.8 points against PT on average. That is why, according to the model, Flávio Bolsonaro is more likely to finish first. The Kalshi market and the Plano Político model put Lula's chance of finishing first at 74% and 70% respectively ^[1][2].

A check on election night (4 Oct 2026). The Lula-minus-Flávio margin is 2.2 points in the poll average and -2.6 points in the model's expectation. If the actual margin turns out no smaller than in the polls, the historical skew has not repeated this time, and our estimate for Lula is too low.

The chance that it is all settled in the first round is 26% (Flávio 18%, Lula 7%). Kalshi prices this outcome at 4% ^[3] — one of our larger disagreements with the market: the model allows for large polling errors and therefore sees extreme outcomes more often.

3 Scenarios: “if X happens, Lula's chance is Y”

We do not estimate the probability of the events themselves — only what happens to the chances if an event occurs. The size of each shift is taken from a real, measured past case.

Why the “polls miss as they did…” rows sit below our range. In those rows the polling error is taken exactly as it was in those years, with no softening and in both rounds at once. The main estimate allows for the error to be smaller this time, or even to run the other way, as in 2006. If history repeats itself in full, Lula's chance would be roughly 19%.

Where the size of the scandal comes from. After audio recordings of Flávio with the banker Vorcaro were published (13 May 2026), Lula gained an average of 3.4 points in head-to-head polls across the 6 pollsters with a poll both before and after ^[4]. Over the same period Flávio's support fell most visibly among evangelicals ^[5]. In the calculation the shift is applied to both rounds — to the first-round gap and to the runoff head-to-head polls — which is why the chance moves so much.

What it would take for the chances to even out. This time PT would have to fall short of its polls by noticeably less than usual. The first-round miss (result minus polls on PT's margin over its rivals) would have to be no lower than -0.5 points — this happened in 0 of 6 cases. The miss of the runoff head-to-head polls would have to be no lower than -0.1 points — this happened in 1 of 5 cases.

4 Our model versus betting markets and other forecasts

In our model the favourite is Flávio Bolsonaro; on Polymarket, the largest market, the favourite is Flávio Bolsonaro. Our estimate for Lula is below the market's, and on the question of who finishes first we part ways with the markets and with other models. This does not prove the market wrong: our first-round forecast rests on a correction that history has confirmed (see “How we calculated”), while the headline chance depends on the runoff correction, which history cannot test. A betting-market price is not an exact probability either: it is shaped by fees, traders' tendency to overpay for long shots (the favourite–longshot bias) and the size of the market.

5 Calendar: what to watch

• 1 Oct 2026 — the last first-round debate, on TV Globo, was cancelled: Lula declined in advance, Flávio pulled out of it after the TSE ruling ^[11][12].
• 3 Oct 2026 — the final wave of polls (Datafolha, Quaest, AtlasIntel and others) ^[13]. We will update the forecast on the morning of election day.
• 4 Oct 2026 — first round. In the evening: the result, and the election-night rule (see above).
• 11 Oct 2026 — runoff debate on Record ^[11].
• 25 Oct 2026 — runoff ^[14].

6 Sources

All data come from public sources. This is an estimate of probabilities, not investment advice.