More than 300,000 people must prove their eligibility for subsidies under the Affordable Care Act, or they could lose coverage next month. The sticking issue is whether the individuals have proof they are living in the U.S. legally. Already, some 450,000 cases with citizenship or immigration discrepancies have been resolved, and an additional 210,000 are “in progress,” according to the Obama administration. However, 310,000 people have not responded to calls, letters, or emails about the inconsistencies. Another round of letters went out Tuesday. If the recipients fail to act, the administration says, they will lose coverage by Sept. 30. Roughly half the cases are in Florida and Texas.