Melissa McHenry, director of external communications for AEP, says, "The proposal is disappointing. After receiving millions of comments, the EPA still proposed a rule that effectively eliminates coal as an option for a new powerplant by setting an emissions limit significantly lower than any existing coal-fueled generating technologies can achieve."

But McCarthy challenged arguments that the proposal would eliminate new coal-fired plants. "This proposal, rather than killing coal, sets a path forward for coal to be part of the diverse mix of the energy supply," she said.

The proposal could face opposition from Republican lawmakers and some Democrats in coal-mining states. Ed Whitfield (R-Ky.), chairman of the House energy and power subcommittee, says he will work to prevent the regulation from going into effect.

Legal challenges to the proposal are almost certain, says Byers. However, Sen. Tom Carper (D-Del.) stresses that, as chairman of the environment and public-works subcommittee on clean air, he would work with EPA to "reject efforts to delay these type of common-sense regulations that are required by law."

The proposed rule will go through a public-comment period and is expected to become final by fall 2014. In the meantime, EPA will begin working on regulatory guidance for air emissions from existing powerplants, due next June.