**Marcelo Castro (MDB) holds the strongest position in the Piauí Senate race for one of the two seats, backed by consistent polling leads through September 2026 and alliances aligned with the state government.** Multiple surveys, including AtlasIntel in early September and Datafolha in mid-September, place him at or near the top in consolidated first- and second-vote intentions, often ahead of or tied with Ciro Nogueira (PP) and Júlio César (PSD). Castro benefits from incumbency, name recognition as a sitting senator, and visible local infrastructure investments. Nogueira, the other incumbent seeking re-election, trails in most recent readings despite his national profile and PP base, while Júlio César remains competitive in the second slot. The field includes roughly 20 registered candidates for the two vacancies, with the remainder drawing minimal support and fragmenting the vote. Election day is October 4, leaving limited time for late shifts, though high undecided shares in earlier polls and the dual-vote system keep the race dynamic among the top three. Trader pricing aligns with these polling patterns favoring Castro for first place.