The fundamental issue is, is carbon dioxide an air pollutant? This is not so much about interpretation of the Clean Air Act IMO, as it is about peoples' opposing views on carbon dioxide as a significant contributor to climate change.

Those who believe carbon dioxide absolutely is contributing to climate change and harming our planet would likely classify carbon dioxide as an air pollutant and thus believe the EPA has the power to regulate it. Those that believe otherwise are not going to consider carbon dioxide an 'air pollutant' and thus not subject EPA regulation.