Tuesday's two hours of Supreme Court oral arguments on ObamaCare's individual mandate were rough-going for the government and its assertions of unlimited federal power. Several Justices are clearly taking seriously the Constitution's structural checks and balances that are intended to protect individual liberty.

Solicitor General Donald Verrilli faced aggressive questioning from Justices Anthony Kennedy and Antonin Scalia and Chief Justice John Roberts, the trio pegged as possible swing votes in favor of the mandate to buy...