U.S. to propose first climate limits on power plants WASHINGTON (Reuters) - The Obama administration will propose as soon as Tuesday the first ever standards to cut carbon dioxide emissions from new power plants, sources involved in talks on the matter said - a move that is likely to be hotly contested by Republicans and industry in an election year. The Environmental Protection Agency is expected to propose the long-delayed rules, known as New Source Performance Standards, that would effectively limit emissions of all new U.S. power plants to those of efficient natural gas plants. "Tuesday could be the day," said one source close to the administration. Opponents of the rules, which include Republicans and some Democrats from energy-intensive states, say they will prevent companies from building new coal plants since coal plants emit roughly double the carbon dioxide as natural gas. The EPA's overall clean-air efforts have divided the power industry between companies that have moved toward cleaner energy, such as Exelon and NextEra, and those that generate most of their power from coal, such as Southern Co and American Electric Power. The sources said, however, that coal plants could add equipment to capture and bury underground for permanent storage their carbon emissions. The rules would likely give any new coal plants time to get those systems running, by requiring that they average the emissions cuts over decades, they added. Record low prices for natural gas and the looming air rules have pushed many companies to put older coal plants into retirement. The EPA is moving forward on the climate rules, which do not need to be approved by Congress, after a wide-ranging climate bill died in the Senate in 2010. Environmentalists who were stung by President Barack Obama's decision last September to delay a major smog rule, cheered the prospect of performance standard rules they say will help protect the country from climate change. "The bottom line for our country is that cleaner power will cut harmful carbon dioxide pollution, protect our children and help secure a safe prosperous future," said Vickie Patton, the general counsel for the Environmental Defense Fund. The rules are expected to affect only new plants, not modified plants, which would be a concession to industry. Existing plants would not be included, but the new proposals could set the stage for the EPA to regulate them in the coming years. The EPA's clean air chief Gina McCarthy has said there's no guarantee the rules will be finalized before the November 6 election, which means they could be more easily overturned if Obama lost the election. The EPA did not immediately comment on when the rules would be proposed. Additional reporting by Jeff Mason; Editing by Edwina Gibbs