The opposition to a proposal by the U.S. Environmental Protection Agency to use the Clean Air Act to regulate greenhouse gas (GHG) emissions continues to grow.

The American Legislative Exchange Council (ALEC), a nonpartisan association of state lawmakers, and the U.S. Chamber of Commerce, both based in Washington, have weighed in on the matter, as has the Steel Manufacturers Association (SMA), which said it would "make a mess" of the Clean Air Act's regulatory framework.

ALEC sent a letter to U.S. Senate Majority Leader Harry Reid (D., Nev.)...