In our constitutional republic, the Congress of the United States is the legitimate legislative branch of government, charged with making the laws. A decision to adopt any national global warming program is an enormous one, with hundreds of billions of dollars and personal liberties at stake. This is simply not something that ought to be done through the back door via an unelected, unaccountable agency.
As with all executive power grabs, the EPA ultimately can only do what Congress allows. Voters must constantly remind their elected representatives that they expect them not only to oppose bad laws but to step in and stop the executive branch when it oversteps its bounds.