The Mexican government is warning that Texas’ denial of birth certificates for U.S. children born here to undocumented immigrants stands to imperil the relationship between Mexico and the Lone Star State. The concern was raised in an amicus brief filed Monday evening to lend support to immigrants parents who sued Texas after being denied birth certificates for their U.S.-born children, even after showing their “matrículas,” the ID cards issued by the Mexican consulate to undocumented immigrants.