During his election campaign, Obama promised he would allow states to each dictate their own rules towards greenhouse emissions. This puts the administration at odds with the automakers, which argue that it's a federal responsibility (along the same lines as regulating fuel economy). Most in the industry feel that having state-mandated greenhouse gas limits would add unnecessary costs to vehicles, and create problems for dealers located near state lines. The states, however, want the right to impose their own regulations. We've been watching this exhausting cage match for years, and the conflict doesn't show any signs of waning.
[Source: Automotive News - Sub. Req.]