LOS ANGELES Proposed federal measures, including fee hikes and online filing requirements, threaten to hinder the ability of some legal immigrants to become U.S. citizens, immigrant rights groups warn. Last week, advocates sent a joint letter to the U.S. Bureau of Citizenship and Immigration Services, expressing concern about application fee increases that could double to $800, a "digital barrier" of a mandatory online filing system, extensive new paperwork and a revised history and civics test they fear could be more difficult. The coalition of more than 230 religious, labor and immigrant rights groups said the rules would disproportionately hinder Mexican...