As keen supporters of Brazil’s President Jair Bolsonaro claim polls signaling he will lose reelection can’t be trusted, an unlikely proxy has emerged: towel sales. Cashing in on skepticism of pollsters ahead of the October election, some street vendors have begun using scoreboards to track sales of towels bearing the faces of far-right Bolsonaro and his rival, former President Luiz Inácio Lula da Silva of the leftist Workers’ Party. The boards have become a social media sensation in recent days. The trend is dubbed DataToalha, a pun alluding to the country’s most prominent pollster, Datafolha (“toalha’’ is Portuguese for towel.) At one stand in Rio de Janeiro’s city center Wednesday, Lula led 43 to 5. Vendor Jose Lucas da Silva, 28, said sales have soared and his scoreboard is drawing crowds and comments from passersby. “It’s a joke, a joke that’s working out!” he said. “Whoever wants to participate just needs to buy!” Political analysts have said that gap will narrow...