Even if ObamaCare survives Supreme Court scrutiny next spring, its trials will be far from over. That's because the law has a major glitch that threatens its basic functioning. It's so problematic, in fact, that the Obama administration is now brazenly trying to rewrite the law without involving Congress.

The Patient Protection and Affordable Care Act offers "premium assistance"—tax credits and subsidies—to households purchasing coverage through new health-insurance exchanges. This assistance was...