Under the Clean Air Act, the EPA has the power to mandate states apply “the best system of emissions reductions,” to existing power plants.
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A key concession touted by vulnerable Democrats in the administration’s new carbon pollution standards may provide the greatest legal threat to the controversial new rules, the cornerstone of President Obama’s climate change agenda.

The administration is giving states broad flexibility on how they meet Environmental Protection Agency targets for existing power plants to reduce their carbon emissions 30 percent from 2005 levels by 2030.

Under the rules, states may take actions to reduce pollution that aren’t directly related to power plant emissions. A state could avoid retiring a power plant by investing in cleaner technology, push energy efficiency programs that will cut demand, or invest in wind and solar, according to the EPA.