It was the 2007 SCOTUS ruling(5-4) that CO2 was a pollutant that set the EPA in motion.
EPA has said that they will not use the Clean Air Act regulatory threshold of 250 tons. Instead, they plan to "tailor the regulation" to 25,000 tons, which will limit those being regulated to the very largest emitters(about 14,000)
Those "very largest emitters" will sue EPA saying that EPA does not have the legal authority to change the regulatory threshold from 250 tons to 25,000 tons.
I hate to tell you this, but EPA is trying to stay out of the way of Congress. But EPA has certain legal obligations to the SCOTUS decision. New York University School of Law published a paper earlier this year defining EPA's legal obligations to the court. The paper was entitled "The Road Ahead" and it has been published and republished on numerous websites.
The EPA is strong arming congress.