RIO GRANDE CITY — Judging by the crowd at Las Lomas, a colonia in one of the nation's poorest counties, no one would have guessed that a majority of local elected officials already had endorsed Leticia Hinojosa's opponent for Congress. The event was billed as a forum for the two Democrats, but when it came time to start, the crowd was chanting only, "Leticia! Leticia! Leticia!" "Hola, chulas (Hi, cuties)," the former state district judge told a pair of young girls who greeted her as they would a favorite tía. A representative for her opponent, U.S. Rep. Lloyd Doggett, D-Austin,...