Al Armendariz resigned in April as EPA's administrator for Region 6, which oversees Texas and other states with oil and gas interests. Republicans criticized him for comparing the agency's enforcement policies to how the Romans brought order to rebellious towns during the Middle Ages.
The Sierra Club said in a statement Friday that Armendariz will be the environmental group's senior campaign representative for the Beyond Coal campaign. He starts in mid-July.
The campaign is designed to encourage Texas, Oklahoma and Arkansas to replace coal-fired electricity with renewable energy sources such as wind and sun.