By executive order and through the issuance of a Clean Air Act waiver, Obama plans to allow California to enforce its own state vehicle emissions standards, the nation's strictest and most aggressive. In effect, all cars sold in the US will have to comply with rules set by Arnold Schwarzenegger and the California legislature.
This is a complete reversal of both traditional concepts developed under the commerce clause (one nation requires one set of uniform economic regulations, as set by the national government) and traditional concepts of federalism (where possible, each state gets to do its own thing). Obama plans to empower one state's government to set emissions standards for all fifty states.
In fact, the California standards are a good idea. But, they should be federalized by the passage of federal legislation, not through a power grab of questionable constitutionality.