PL and UPB lead the implied probabilities for the largest caucus in Brazil’s October 4, 2026, Chamber of Deputies election because they enter as the two biggest current blocs under the open-list proportional system with D’Hondt allocation and a 2% threshold. PL, with roughly 97–99 seats after the early-2026 party-switching window, benefits from consolidated national infrastructure, candidate recruitment, and alignment with the right-wing presidential campaign of Senator Flávio Bolsonaro. UPB’s roughly 98–100 seats reflect its position as the core of the pragmatic Centrão, which maintains strong state-level organization and access to resources despite some losses in the same window. High incumbent re-election rates and the fragmented field for smaller parties reinforce the duopoly in trader pricing, with recent presidential polling tightness sustaining mobilization for both leading groups.