Last month, the District of Columbia Court of Appeals heard arguments in a case that strikes at the very heart of the Affordable Care Act's ability to make insurance coverage affordable nationally. Along with three other cases filed in Virginia, Oklahoma, and Indiana, Halbig v. Sebelius asks whether the Affordable Care Act permits the federal government to pay premium tax credits for health plans sold through the federally facilitated marketplace.

Thirty-six states rely on the federal marketplace for individual coverage; the rest operate their own marketplaces. More than 7 million people have enrolled in marketplace health plans during the initial open enrollment period that closed March 31. According to the U.S. Department of Health and Human Services' March 2014 enrollment report, 83 percent of enrollees to date have received subsidies, with the number of subsidized enrollments in the federal marketplace (85%) surpassing those in the state-based marketplaces (81%).

Also read:The potential to sink Obamacare?Halbig challenges the legality of an IRS rule permitting payment of federal premium tax credits to eligible individuals regardless of whether they buy coverage through a state-based or federally facilitated marketplace.