Jul. 19--One of the largest power plant operators in Connecticut is joining environmental groups and other Fortune 500 companies in calling for federal legislation to limit greenhouse gas emissions.

NRG Energy announced on Wednesday that it has become a member of the United States Climate Action Partnership, which is seeking to reduce global warming gases by a mandatory limit on emissions and a system of trading pollution credits. The partnership, which began in January, already includes Fairfield-based General Electric, major automakers, energy companies and the Natural Resources Defense Council and the group Environmental Defense. "The time is now for decisive action to address climate change and decisive action requires clear and unequivocal leadership," said David Crane, president and CEO of NRG.

Stamford-based Xerox also announced its membership in the partnership Wednesday, becoming the first technology company to join. After years of complaints about emissions from NRG's plants in Middletown, Norwalk, Milford and Montville -- four of the so-called "Sooty Six" -- the legislature passed tougher emission regulations in 2002. The law set new standards for unhealthy pollut nts such as sulfur dioxide. NRG fought passage of the bill. Now, as concern about global warming grows, the focus has turned to carbon dioxide emissions. By joining the climate action group, NRG is following a developing trend in which major companies are switching from disputing the effects of global warming to advocating regulations for carbon dioxide emissions that seem inevitable, environmentalists said.

"We have shifted from if we are going to tackle global warming to when and how," said Jon Coifman of the Natural Resources Defense Council in New York City. Businesses see benefits in becoming involved in the movement beyond improving the environment. Power companies like New Jersey-based NRG, which owns plants in several states, will probably want clear rules on emissions as they plan new power plants and efurbish existing plants. The sooner power companies know what the rules are, the less risk they face when investing billions in plants that will last for decades, said Tony Kreindler, a spokesman for Environmental Defense.

"Business likes stability. The sooner they know the rules of the road, the better for them," he said. Right now, power companies face the possibility of a patchwork of regulations. Connecticut and other states in the Northeast have the Regional Greenhouse Gas Initiative that regulates carbon dioxide emissions from power plants starting in 2009. Californ a is developing its own system, and Florida recently announced a program, as well. "Any business entity would like to see one set of requirements that deals with any set of competitors," said Jessie Stratton of Environment Northeast, a regional environmental advocacy and research organization with offices in Connecticut.

The pressure seems to be having an effect on Congress. Kreindler said legislation could reach the floor of the House and Senate by the fall. Exactly how the system works and how strong the limits are will determine the effectiveness of the law, Kreindler said. Contact Mark Peters at mrpeters@courant.com.