Bloomberg has been a vocal advocate for killing coal-fired power. He said health problems from pollution and climate change-exacerbated events like Hurricane Sandy have fomented growing recognition that coal “doesn’t deserve” its reputation as a cheap energy source.

For now, coal is still the leading supplier of electricity. It accounted for 37.4 percent of the nation's electricity mix last year, according to U.S. Energy Information Administration statistics released Tuesday.

But that figure also represented a 12.5 percent drop from the previous year. Natural gas, on the other hand, spiked 21.4 percent to provide 30.4 percent of the nation's power.

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Bloomberg attributed coal's struggles to the Environmental Protection Agency's regulations to curb emissions in the name of public health, grassroots environmental efforts and low natural-gas prices.

But Bloomberg noted the U.S. still has a long way to go to achieve “the sustainable future that we all want.”

Coal-fired power plants still account for 40 percent of the nation’s carbon emissions, he noted. And while one-sixth of those facilities are slated for retirement, he said that still leaves a majority standing.

Imposing emissions rules on existing coal-fired plants could be one way to shutter more coal-fired generation, which green groups are pushing Obama to do.

Bloomberg said public health-advocates and those concerned about climate change are ready for a long "regulatory trench war" regarding coal-fired power.

Industry and congressional Republicans, however, have pushed back against many of the EPA emissions regulations. They say the rules are economically burdensome, and have accused the White House of targeting the coal industry.

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While industry and many Republicans have fought to protect coal-fired power, Bloomberg has had his hands in many areas in an attempt to end it.

Bloomberg Philanthropies awarded a $50 million grant to Sierra Club’s Beyond Coal Campaign, which aims to retire one-third of the nation’s coal-fired fleet by 2020.

Bloomberg Philanthropies also gave a $6-million grant to the Environmental Defense Fund to work on hydraulic fracturing, or fracking, regulations.

The process injects a high-pressure mixture of water, sand and chemicals into tight-rock formations to unlock oil-and-gas deposits deep underground. Green groups are concerned the practice could contaminate drinking water, while industry contends it’s safe.

Bloomberg, who touted natural gas’ lower carbon content compared with coal, said companies should work with environmental groups to design regulations with which they can live.