Power company Exelon Corp. (EXC) on Monday joined a stream of companies quitting the U.S. Chamber of Commerce over the chamber's stance against federal climate-change legislation.

The decision by Exelon, the nation's biggest nuclear power plant operator, follows similar moves last week by utilities PG&E Corp. (PCG) and PNM Resources Inc. (PNM), and highlights a growing rift in the nation's power sector and in other industries over climate policy. The U.S. government is under pressure, both from other countries and from U.S. states, to commit to reductions in greenhouse-gas emissions, particularly with the approach of a key global climate-change treaty summit in Copenhagen in December.

Chicago-based Exelon said the U.S. government needs to set climate-change policy promptly so companies can "put a price on carbon" and figure out how much it will cost to cut their emissions. The U.S. House of Representatives in June passed a landmark bill that would require the U.S. to cut greenhouse-gas emissions 17% from 2005 levels by 2020, and create a market-based program called cap-and-trade in which companies could buy and sell the right to emit carbon dioxide.

"The carbon-based free lunch is over," Exelon Chairman and Chief Executive John W. Rowe said in a statement. "But while we can't fix our climate problems for free, the price signal sent through a cap-and-trade system will drive low-carbon investments in the most inexpensive and efficient way possible."

The companies' departures are unlikely to change the Chamber's position on climate-change policy, said David Chavern, the group's chief operating officer. He added that although the Chamber opposed the House bill and it disagrees with plans by the U.S. Environmental Protection Agency to begin regulating greenhouse-gas emissions, the group isn't opposed to U.S. climate-change legislation.

"Congress should do everything it can to promote and incentivize technology development and other policies that allow us to control carbon in ways that don't trash the economy," Chavern said.

Exelon, PG&E and PNM all operate nuclear power plants and emit far less carbon dioxide than some of their peers, particularly companies that operate large fleets of coal-fired power plants. Coal plants produce roughly twice the greenhouse-gas emissions of similarly sized natural gas-fired plants. Nuclear power plants emit almost no greenhouse-gas emissions.

Despite their differences, U.S. power companies, represented by the lobbying group Edison Electric Institute, banded together in support of the climate-change legislation that passed the House.

The U.S. Chamber opposed that bill, sponsored by Reps. Henry Waxman, D-Calif., and Edward Markey, D-Mass. It also recently suggested that the U.S. hold a "Scopes-like" trial to debate evidence that climate change is man-made, in response to a proposed finding by the EPA that global warming poses a danger to public health.

The EPA's proposed finding and potential greenhouse-gas rules are in response to a 2007 Supreme Court ruling that directed the agency to determine whether greenhouse gases are pollutants that should be regulated under the Clean Air Act.

California Gov. Arnold Schwarzenegger last week criticized federal lawmakers for delaying action on climate-change legislation and urged them to ignore what he called "naysayers" who oppose such legislation.

California's 2006 climate-change law requires energy and other companies to cut their greenhouse-gas emissions starting in 2012. Ten Northeastern states, including New York and New Jersey, require power companies to cut their greenhouse-gas emissions under a program called the Regional Greenhouse Gas Initiative.

U.S. Sen. Barbara Boxer, D-Calif., is expected to unveil a climate-change bill shortly.

-By Cassandra Sweet, Dow Jones Newswires; 415-439-6468; cassandra.sweet@dowjones.com