US Environmental Protection Agency Administrator Gina McCarthy has told Senate Environment and Public Works Committee that the agency's plan to require mandatory cuts in carbon pollution from power plants does not overstep its authority or stretch the parameters of the Clean Air Act, as some of its political opponents have claimed.

The committee's top Republican member, Senator David Vitter of Louisiana, questioned whether EPA was directing states to achieve "questionable emission reduction targets from a limited menu of economically damaging and legally questionable options."

McCarthy said the EPA was not expanding its authority in crafting the proposal, adding that each state can comply with the rules as it best sees fit. To mollify lawmakers who contend the EPA is waging war on fossil fuels, she said coal and natural gas will remain a part of the US energy mix.

The EPA leader said the Obama administration's plan builds on action already under way to modernize aging plants, increase efficiency and lower pollution, and paves a more certain path for conventional fuels in a clean energy economy.

The rule sets the country's power sector on a course to reduce carbon dioxide emissions by an average of 30% below 2005 levels by 2030. Each state will have a customized target to reduce the carbon intensity of electricity production below 2012 levels by 2030.

In a letter to McCarthy, manufacturing and energy industry lobby groups aired concerns. The Partnership for a Better Energy Future, a coalition of business organizations formed in January to lobby against the EPA's plans, said the proposal would be too disruptive to the U.S. energy system and should be dumped.

The group also hinted at potential legal action, arguing that the EPA had overstepped the bounds of the Clean Air Act to draft its complex rule.

"Even more fundamentally, the proposal is based on a flawed interpretation of the Clean Air Act. We therefore urge EPA to go back to the drawing board on this rule," the coalition wrote.

McCarthy said the proposal was not a radical approach to slashing carbon emissions, saying the proposal "is designed to be moderate. "We will get significantly more benefit than we are requiring."