**Janaina Riva (MDB) leads the market for second place in Mato Grosso’s October 4 Senate contest at 67.5% implied probability, consistent with recent polling that places her ahead of the field for the state’s second seat.** Multiple September surveys, including Paraná Pesquisas (Sept 15–17) and earlier Real Time Big Data and MT Dados releases, show ex-governor Mauro Mendes (União) commanding first place with over 50% support while Riva holds a clear second at roughly 32%, ahead of José Medeiros (PL) at 23% and Carlos Fávaro (PSD) at 19%. Riva benefits from steady gains in second-preference votes, stronger performance among women, and her established profile as a three-term state deputy heading the “Coração da Gente” coalition. Lower-probability names such as Medeiros, Fávaro, and Pedro Taques trail in both first- and second-choice metrics, with no major late shifts reported in the final weeks before voting. Trader pricing reflects this polling hierarchy for the second of the two seats.