WASHINGTON - Exxon Mobil Corp. and other large oil companies are backing a carbon tax proposal put forward earlier this year by a group of former Republican leaders including James A. Baker III, the former secretary of state and a Houston attorney. The Climate Leadership Coalition, a group that includes Baker, former Treasury Secretary Hank Paulson, and former Secretary of State George Shultz, announced a list of "founding members" Tuesday that includes Exxon Mobil, BP, Shell, Total, General Motors and Johnson & Johnson. "We support @TheCLCouncil as a founding member and are working to support its policy development process," Exxon...