"It's by far the most ambitious and comprehensive administrative action on climate change we've ever seen proposed," said Gene Karpinski, president of the League of Conservation Voters. "He has said, 'If Congress doesn't act, I will.' It's clearly time to act, and he's setting out a bold, ambitious, comprehensive plan for what he can do without needing to rely upon Congress."

Is this really a significant blow against climate change, then, or just the best advocates could hope for given the circumstances? Karpinski insisted it's a major move by any standard: Existing power plants generate about 40 percent of the U.S.'s total carbon emissions, and depending on how the rules are crafted, the new push could achieve a 17 percent reduction in greenhouse gas emissions by 2020 -- the same goal cap-and-trade was meant to hit. (Cap-and-trade, however, would have kept reducing emissions beyond that horizon, something this action can't achieve.)

"If [EPA does] as strong a proposal as they can do on power plants, that can be a huge bite out of pollution from the single biggest source," Karpinski said.

In political terms, the environment has receded mightily as an issue since the heady days when Nixon created the EPA. Successive versions of the Clean Air Act passed with huge bipartisan majorities in Congress under two Republican presidents, Nixon and George H.W. Bush. These days, not only is climate legislation a nonstarter, Republicans see the very idea as a potent political weapon, and even many Democrats are leery of raising an issue many see as a job-killer.

But despite the impression of a looming, perpetually unaddressed crisis, Karpinski said things are actually getting gradually better, at least where U.S. emissions are concerned. New mileage standards for cars and trucks have already been issued, another major but largely unheralded administrative action. States and localities have taken the lead in pushing renewable energy and pollution controls. "Our air is getting cleaner, our water's getting cleaner," Karpinski said. "We're beginning to bend the curve on carbon pollution, but we need to do better."

Recently, the U.S. fell from its long-held spot as the world's No. 1 polluter, ceding its place to China. Part of the goal of Obama's new push will be to set an example for the rest of the world ahead of a major 2015 climate summit, Karpinski said. And as for Keystone -- which a senior administration official said Monday was "not yet ready for a decision" -- environmentalists seem willing to ignore that for the moment. "Keystone is down the road," Karpinski told me. "[Tuesday's speech] is about huge steps forward."

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