But the biggest fights are among energy producers. They have spent more than $200 million in the first half of the year on lobbying efforts in Washington, according to the Center for Responsive Politics, a nonpartisan research group, up from $174 million in the same period last year.

“The fact that the lobbying is so fast and so furious is a positive sign that this thing is moving along,” said Mark Brownstein, a managing director at the Environmental Defense Fund and an advocate of climate legislation. “The fact that everyone is rushing to Washington tells you people believe it is real.”

Image Energy executives before a House committee in February. Second from right, J. Larry Nichols of Devon Energy and the American Petroleum Institute, says favoring some fuels had a flaw. Credit... Ruth Fremson/The New York Times

As legislation inches through Capitol Hill, onetime allies in the utility sector, like Exelon, which operates low-emission nuclear plants, and the Southern Company, a big consumer of coal, find themselves on opposite sides of the debate over renewable energy.

Utilities that have access to hydroelectric power or operate nuclear plants tend to favor a national mandate to increase the use of renewable power, because their carbon emissions are relatively low. Many coal-dependent utilities, particularly in the Southeast and Midwest, oppose the provision because they emit more carbon and would have to buy more permits over time.

In past energy policy debates, the oil and gas lobbies were largely united. In 2005, they won incentives for drilling in the Gulf of Mexico. Two years later, after Democrats had taken control of Congress, producers were unable to block a huge new mandate for alternative fuels like ethanol and biodiesel, but managed to save valuable oil industry tax breaks that some Democrats tried to end.

Today, each energy subsector, fearing any legislation that might give it a disadvantage, is battling for favor. The gas producers, for example, have formed the American Natural Gas Alliance, which is spending heavily on advertising and lobbying to point out that gas emits roughly half the carbon dioxide of coal. The group has also helped organize its allies in Congress into a new natural gas caucus, with two dozen members.