On March 4 the Supreme Court will hear oral arguments in King v. Burwell, with a decision expected in late June. If the court strikes down the payment of government subsidies to those who bought health insurance on the federal exchange, Republicans will at last have a real opportunity to amend ObamaCare. Doing so, however, will be politically perilous.

The language of the Affordable Care Act states that subsidies should only be paid through state exchanges. The bill’s authors perhaps believed that pressure from citizens and...