Flávio Bolsonaro holds an 87.3% implied probability in the runoff against President Luiz Inácio Lula da Silva due to his narrow first-round victory on October 4 and consistent polling leads of four to six points in subsequent surveys. The senator consolidated support from eliminated right-wing candidates and benefited from voter momentum that shifted undecided and opposition ballots toward him in the days after the initial vote. Lula trails amid elevated disapproval ratings and economic dissatisfaction, while no first-round runner-up has won a Brazilian presidential runoff since the return to democracy in 1985. The October 25 contest remains the decisive factor, with remaining undecided voters and turnout patterns among key regions likely to determine the final margin.