Amid rising power demand and flat U.S. natural gas output, electricity generators are seeking to build power plants fired by abundant coal.

The fuel is cheap compared with other fossil fuels, but emits more pollutants, such as mercury and smog components sulfur dioxide and nitrogen oxides, as well as the greenhouse gas carbon dioxide.

"President George Bush's administration is responding to this growing threat to our national parks by seeking to weaken and rewrite the laws that protect national park air quality," Mark Wexler, the clean air director for the National Parks Conservation Association, a Washington-based advocacy group, said by telephone.