In her confrontation with Bernie Sanders, Hillary Clinton always promises to “build on the successes” of ObamaCare, so allow us to recommend a follow-up question: What would those be, precisely? The entitlement is becoming less stable and less entrenched, not more, as it gets older.

The latest jolt is the $475 million loss UnitedHealth Group booked on the insurance exchanges in 2015, which the largest U.S. mega-insurer by membership expects to rise this year to another $500 million.