The Supreme Court’s just-issued decision restricting the Environmental Protection Agency’s Clean Air Act authority to regulate power-sector greenhouse gases is elevating the importance of the judicial “major questions” doctrine, which imposes strict scrutiny on agency rulemaking. The high court points to its 2000 decision to shoot down FDA’s effort to regulate tobacco outside a clear congressional mandate as a precedent for its decision, but the conservative majority in the EPA case doesn’t fully outline the limits of EPA’s authority. The...