MEXICO CITY -- To commemorate the new year, a mayoral candidate in a small Mexican town sent a Facebook message Sunday morning asking residents to unite to improve society. “We only need maturity, seriousness, and responsibility to face the challenges that confront society,” Adolfo Serna Nogueda wrote. Later that day, Serna was fatally shot outside his home in Atoyac de Alvarez, along the Pacific Coast in the western state of Guerrero. Serna, a member of the ruling Institutional Revolutionary Party, was one of at least five politicians killed in the past week in Mexico on the eve of an important election year. Two days earlier, the mayor of another Guerrero town, Petatlan, about two hours north along the coast, was killed while eating with friends at a restaurant. And the day before that, a state congressman from Jalisco was gunned down while driving with his son. A former state congressional candidate and a town council member also were killed in the past week.