The U.S. Supreme Court's surprise announcement on November 7 that it would hear King v. Burwell struck fear in the hearts of supporters of the Affordable Cara Act (ACA). At stake is the legality of an Internal Revenue Service (IRS) rule extending tax credits to the 4.5 million people who bought their health plans in the 34 states that declined to establish their own health insurance exchanges under the ACA. The case hinges on enigmatic statutory language that seems to link the amount of tax credits to a health plan purchased "through an Exchange established by the State." According to the plaintiffs in King, that language means that consumers who buy insurance through federally run exchanges don't qualify for subsidies. The Court's decision to hear the case without a split between appellate courts suggests that at least four justices harbor serious doubts about the IRS rule's validity.