Prioritizing federal environmental regulation as the primary means of achieving dramatic, rapid reductions in greenhouse gas (GHG) emissions may be a strategic mistake. Regulatory mandates, particularly if based upon existing statutory authority, will be vulnerable to legal attack, obstruction, and delay; climate legislation can reduce legal risks and accelerate policy implementation, but only on the margin. Adopting regulatory controls will be immensely resource-intensive for the U.S. Environmental Protection Agency and other agencies. Even with authorizing legislation, regulatory strategies may remain more time-consuming, conflict-ridden, and legally vulnerable than fiscal measures. A carbon tax, in particular, would be more legally secure and administratively easier to implement than regulatory controls on energy use and GHG emissions.