By Moira Warburton WASHINGTON (Reuters) -Centrist U.S. Representative Henry Cuellar was clinging to a razor-thin lead early on Wednesday against progressive challenger Jessica Cisneros in a high-profile south Texas Democratic primary battle that illustrated sharp dividing lines over immigration and abortion rights. The election on Tuesday in a district along the U.S.-Mexico border was the third contest between Cuellar, who has held the seat since 2005, and Cisneros, a 28-year-old attorney who failed to unseat him in 2020 but forced him to a runoff in the state’s March primary this year. A tall…