The plaintiffs say Congress set it up this way to compel states to take responsibility for running their exchanges. But in 2012, the IRS unlawfully extended tax credits to all the exchanges, they argue.

The Obama administration and its allies say Congress always intended to treat the states equally. The law calls on the federal government to set up an exchange for states that refused to, they argued.

A three-judge panel from the U.S. Court of Appeals for the D.C. Circuit is mulling the case and may issue its opinion any day now. No matter the outcome, the losing side is certain to appeal, potentially cuing up yet another Obamacare battle before the Supreme Court.