Exelon Corp. (EXC) on Monday urged lawmakers and industry groups to work toward passing climate-change legislation and said it was quitting the U.S. Chamber of Commerce over the group's opposition to climate-change legislation pending in Congress.

Chicago-based Exelon, the nation's largest nuclear-power plant operator, said the U.S. government needs to set climate-change policy promptly, in part to "put a price on carbon," or allow energy markets to attach a price on cutting one ton of carbon dioxide, or its equivalent.

"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."

Exelon's decision to quit the U.S. Chamber of Commerce follows a similar move by utility PG&E Corp. (PCG) last week. Both utilities have smaller carbon footprints than many 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.

The U.S. Chamber has been a vocal critic of climate legislation pending in the Senate, recently suggesting that the U.S. hold a "Scopes-like" trial to debate evidence that climate change is man-made.

The U.S. Environmental Protection Agency has dismissed the Chamber's proposal, saying that its own proposed finding that global warming poses a danger to public health is based on sound science.

Despite their differences, U.S. power companies, represented by lobbyist group Edison Electric Institute, banded together in support of climate-change legislation that passed the U.S. House of Representatives earlier this year. A similar bill is pending in the U.S. Senate.

Rowe of Exelon said climate-change legislation should focus on conserving energy by increasing efficiency.

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