Almost all of the state's poorest residents will have to show proof of US citizenship to continue getting medical care by July 1, under a little-noticed federal law that could endanger coverage for many, as Massachusetts is trying to expand access to healthcare. Born out of ongoing efforts in Washington to clamp down on illegal immigration, the new federal requirement compels anyone seeking Medicaid coverage to provide a birth certificate, a passport, or another form of identification in order to sign up for benefits or renew them. No such proof is required now. The requirement was tucked into the Deficit...