A maverick former mayor known as El Bronco is mounting a serious bid to become Mexico's first independent candidate to win a governorship, buoyed by voter mistrust of the country's traditional political parties. Waging a social media campaign on a shoestring -- paid for largely with the crumpled bills supporters press into his hands on the stump -- Jaime Rodríguez is shaking up politics in Nuevo León, the conservative northern border state that includes the industrial powerhouse of Monterrey, and jolting politicians nationwide. An opinion poll published Friday in El Norte, Monterrey's leading newspaper, puts Mr. Rodríguez ahead of his...