Brazil’s October 4 first-round turnout of 78.92% establishes the baseline for the October 25 runoff between President Lula and Senator Flávio Bolsonaro. The tight polling contest and intensive mobilization by both campaigns, including recent rallies and the final debate, have kept trader focus on participation levels near that mark, supporting the 78-80% band as the leading outcome. Historical patterns show Brazilian runoffs often match or fall slightly below first-round figures when abstention remains elevated, while the high-stakes contest and voter-transfer dynamics from eliminated candidates could lift or suppress turnout depending on last-minute engagement. The narrow spread between the top two probability buckets reflects ongoing uncertainty over whether mobilization will fully offset typical runoff fatigue.