March 5, 2012. Natural gas, which already is edging aside coal in American electricity generation, would be one of the biggest beneficiaries of a clean-energy mandate for utilities under consideration in Congress this year. Senator Jeff Bingaman, a New Mexico Democrat and chairman of the Energy and Natural Resources Committee, introduced a measure to force electricity companies to use an increasing share of energy produced from “clean” sources over the next two decades. The bill reshapes the energy debate by calling for sources that emit less carbon than coal, a definition that includes natural gas, instead of focusing on zero-emission renewable sources such as wind and solar. While the proposal faces long odds of getting enacted this year, Bingaman’s plan may gain a powerful ally -- and new opposition from environmental groups. Bingaman’s clean-energy mandate may benefit companies like Exxon Mobil Corp. of Irving, Texas and Oklahoma City-based Chesapeake Energy Corp., the two largest producers of natural gas in the U.S.