(Originally published on September 28, 2009)

The U.S. Chamber of Commerce’s opposition to climate-change legislation has cost it another member. Exelon, one of the biggest utilities in the U.S., said this morning it will leave the business lobby because of the latter’s increasingly strident opposition to climate legislation.

The announcement came as chairman and chief executive John Rowe spoke to the American Council for an Energy Efficient Economy about the importance of jumpstarting investment in energy efficiency. The company said in a press release:

Exelon is so committed to climate legislation that Rowe announced during today’s speech that Exelon will not be renewing its membership in the U.S. Chamber of Commerce due to the organization’s opposition to climate legislation.

Mr. Rowe stressed the need for a cap-and-trade program that would give power companies an economic incentive to change the electricity business:

“The carbon-based free lunch is over. 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,” said Rowe. “Putting a price on carbon is essential, because it will force us to do the cheapest things, like energy efficiency, first.”

Exelon’s departure comes on the heels of two other defections from the Chamber: California’s PG&E was first out the door, specifically citing the Chamber of Commerce’s recent call for a “Scopes Monkey Trial” for climate change. That was followed by New Mexico’s PNM, which like Exelon, will let its membership lapse at the end of the year.

For a company such as Exelon, which is heavy on nuclear power, climate legislation makes sense because it would penalize “dirty” electricity such as that generated by coal and reward low-emissions electricity, be that from nuclear power or renewable energy.

Depending on how climate legislation finally plays out in Congress—the Senate will take up the issue again this week—Exelon’s past investments could lead it to reap big rewards. If Congress eventually decides to dole out greenhouse-gas emissions permits based on how much electricity companies generate, then utilities such as Exelon and PG&E would stand to gain a lot more currency.