Rather than looking at peak periods of pollution from a power plant, the proposed rule would instead use annual averages. An agency spokeswoman says the rule won't result in more pollution.

But Republican Sen. Lamar Alexander of Tennessee is building a bipartisan coalition to oppose the rule. More than 20 coal-fueled power plants are under construction in Wyoming and 13 other states and more than 100 planned.

The National Parks Conservation Association says ten national parks are most at risk from new plants including Wind Cave and Badlands in southwest South Dakota.