I won't recall all the details but the fed argue that emissions standards are the domain of the Environmental Protection Agency (which ruled at the beginning of Bush Jr that CO2 wasn't a poison), and they also claim that emission standards are only possible through fuel economy which is the domain of congress (and just recently, congress refused to increase meaningfully fuel economy standards). The courts have so far ruled for the states but the states will need EPA approval and I have also a hard time imagining that push come to shove the Federal Society types on the supreme court will let corporations lose that battle.

Also, 12 states have followed California's lead, which represents over half the economy of the US, but I seem to recall that no more states could adopt this type of legislation because of some technicality. Although I am not sure how binding this is. These 13 US states would be the only ones, including Europe, with mandatory emission standards, although I haven't compared actual emissions in Europe and in the US.

The Union of COncerned Scientist, based in Cali, has quite a bit of info on the status and history of the legislation