Washington ? The Bush administration on Tuesday ordered power plants to cut mercury pollution from smokestacks by nearly half within 15 years but left an out for the worst polluters.

The Environmental Protection Agency said the cuts would help protect pregnant women, women of childbearing age and young children from a toxic metal that causes nerve damage.

Critics said the arrangement fell far short of what was needed, and they promised to fight it.

The nation’s 600 coal-burning power plants release 48 tons of mercury pollution a year. That is expected to decrease to 31.3 tons in 2010, 27.9 tons in 2015 and 24.3 tons in 2020.

Forty percent of mercury emissions come from power plants, but those emissions have never been regulated as a pollutant. EPA regulates mercury in water and from municipal waste and medical waste incinerators.

EPA faced immediate political and legal opposition. Senators, environmentalists and public health advocates said EPA failed to do all the Clean Air Act requires.

They said EPA favored industry by setting a nationwide cap on allowable pollution and then allocating a specific amount to each state — and, in a few cases, Indian tribes that own power plants. The states then set limits on specific plants. Those that exceed the limit could buy pollution “credits” from plants emitting less pollution than they’re allowed.