Taking a historic step to fight climate change, the Environmental Protection Agency today proposed a plan that aims to slash carbon dioxide emissions from existing power plants 30% by 2030 and could accelerate the nation's shift away from coal.

"By leveraging cleaner energy sources and cutting energy waste, this plan will clean the air we breathe while helping slow climate change so we can leave a safe and healthy future for our kids," said EPA Administrator Gina McCarthy, adding it will spur innovation and create jobs. She said the plan will give states flexibility to lower power plant emissions, setting goals tailored to their circumstances.

Yet the controversial proposal, a major part of President Obama's climate initiative, will set a national target of lowering these heat-trapping CO² emissions — from 2005 levels — of 30% by 2030. The EPA says that reduction amount is equal to the emissions from powering more than half of U.S. homes for one year.

Thwarted by Congress' inability to pass a bill to lower carbon emissions, Obama is pushing his own approach. Last June, he asked the EPA to use its authority under the Clean Air Act to limit power plant emissions, which account for the largest share — nearly 40% — of total U.S. emissions. Coal-fired facilities will be hardest hit, because they emit more carbon than other power plants.

The rule, expected to trigger legal challenges, will not take effect for at least two more years. Obama has asked the EPA to finalize it in June 2015, after which the states will have at least a year to craft their plans. If states balk at submitting them, the EPA could step in with its own version.

Opponents are already lining up against the proposal. Today, Rep. Nick J. Rahall, D- W.V., said he will work with his state's GOP colleague, Rep. David McKinley, to introduce legislation to stop both the new rule for existing power plants as well as one proposed last year for future plants.

"This new regulation threatens our economy and does so with an apparent disregard for the livelihoods of our coal miners and thousands of families throughout West Virginia," Rahall said.

Last week, the Chamber of Commerce released a report saying such regulation could raise consumer prices for electricity, kill jobs and slow economic growth. In the GOP Saturday radio address, Wyoming's Sen. Mike Enzi said the Obama administration has "set out to kill coal and its 800,000 jobs." If it succeeds, he warned, "we'll all be paying a lot more money for electricity — if we can get it."

McCarthy said critics have "time after time ... cried wolf to protect their own agenda." She said their dire predictions about the economic costs of reducing urban smog in the 1960s and acid rain in the 1990s have been proven wrong.

"We can innovate our way to a better future," McCarthy said. "From the light bulb to the locomotive; from photovoltaic cells to cellphones, America has always turned small steps into giant leaps."

Obama said Saturday that the proposal will reduce air pollution, improve health and spur a clean energy economy that can be "an engine of growth." He spoke from the Children's National Medical Center in Washington, D.C., after visiting with kids being treated for asthma and other breathing problems that he said are aggravated by dirty air.

The administration says its proposal will save more than $90 billion in climate and health benefits and will avoid up to 100,000 asthma attacks and 2,100 heart attacks annually.

The EPA says it expects coal, which now provides 37% of the nation's electricity — down from 52% in 2000 — will still provide 30% of U.S. power by 2030. It says the increasing retirements of coal-fired plants, which now average 42 years, are because of economics such as the plunging price of natural gas — not its proposal.

Coal-fired power plants have already been closing. DOE data indicate the number has fallen from 633 in 2002 to 557 in 2012 and it expects 60 gigawatts of coal-fired power — one-fifth of total U.S. coal capacity in 2012 — will retire by 2020.

In contrast, natural gas has seen its share of U.S. electricity generation nearly double, from 16% in 2000 to 30% in 2012. McCarthy said U.S. wind energy has tripled and solar has grown ten-fold since 2009 — two sources that she says can help states meet carbon emissions cuts.

"This rule would accelerate that shift" away from coal, says Kyle Aarons, a senior fellow at the Center for Climate and Energy Solutions, a non-profit group.

The carbon limits could lead to "draconian changes" in the U.S. energy mix, says Karen Harbert, president of the U.S. Chamber of Commerce's Institute for 21st Century Energy.

As coal use and total energy consumption has declined, so too have total U.S. carbon emissions, which have fallen about 10% since 2005. DOE forecasts a slight rise in the near future without new emission caps. In 2009, Obama pledged to lower U.S. emissions 17% below 2005 levels by 2020.,

Power plants account for roughly one-third of all U.S. greenhouse gas emissions, While plants are limited in how much arsenic, mercury, sulfur dioxide, nitrogen oxides, and particle pollution they can emit, they currently face no federal limits on carbon releases.

Some states that rely heavily on coal might struggle more than others to meet the EPA limits. Kentucky, Wyoming, West Virginia, Indiana and North Dakota have the highest carbon emission rates while Idaho, Vermont, Washington, Oregon and Maine have the lowest, according to a May report co-authored by Ceres, a non-profit group that promotes corporate sustainability.

Dan Bakal, Ceres' director of electric power, says coal-reliant states "have the most opportunities to reduce emissions, because there are many options they've not yet deployed." Given the state-specific goals in the EPA plan, he says the reductions are "achievable."