Two years after upholding the law by a single vote, the justices are weighing whether to hear a Republican-backed appeal that would block people in 36 states from getting tax subsidies to buy insurance. The justices are scheduled to discuss the matter tomorrow, with an announcement coming as soon as Nov. 3.

The tax credits have implications well beyond the 4.6 million people who receive them in those states. A high court decision against the administration would have ripple effects, undercutting other parts of the Affordable Care Act and potentially destabilizing insurance markets across the nation.