The three plants - one a public utility, the others private businesses - would generate about 3,900 megawatts of power, enough for more than 1 million households. They also would emit toxic substances that would quickly cross the state line and head for Utah cities.

"These plants present a direct threat to the airshed here in Utah," said Dave Becker, staff attorney for Western Resource Advocates in Salt Lake City. "They certainly shouldn't be built in the way they are proposed."

In a letter dated Monday, Reid, the four-term Nevada Democrat, told Nevada Power/Sierra Pacific Resources, the LS Power Group, Dynegy Inc. and Sithe Global Power that he would use his considerable congressional power to block their construction plans.

"Because I believe that developing renewable energy in Nevada is far preferable to coal for the sake of our economy, public health and the environment, I will use every means at my disposal to prevent the construction of new coal-fired power plants in Nevada that do not capture and permanently store greenhouse gas emissions," he wrote in the letter, released by conservation groups and media outlets on Thursday.

Reid also said Nevada's energy needs could be met largely through new renewable energy and efficiencies.

The senator's letter came after a grassroots letter campaign started by the Sierra Club and other advocacy groups, said the Nevada Sierra Club's Lydia Ball.

The National Park Service opposes the plants proposed for the Ely area. A coal-fired power plant proposed for White Pine County would pollute Great Basin National Park, ruin views and kill trout that make the park so popular, the Park Service said in comments to the Nevada Department of Environmental Protection.

The plants also would emit mercury that would add to the thousands of tons of emissions gold mines near Elko release in the prevailing westerly winds that cross Salt Lake City into southeastern Idaho.

Currently available coal-burning technologies can remove 50 percent to 90 percent of mercury. The Sierra Pacific and Sithe Web sites say the plants would emit no more than is allowed under U.S. Environmental Protection Agency rules.

But the plants won't capture or sequester carbon dioxide.

"That is the wrong technology when we consider climate change brought on by global warming," said Lawson Legate, southwest regional director for the Sierra Club.

The Toquop plant planned for the Mesquite area would be about 80 miles from St. George. "People are going there specifically [for] the quality of life and the quality of air," Becker said. Reid's opposition to the plant "can only be good for Utah, and serves as an important example for the Utah congressional delegation."

Coal creates about 95 percent of Utah's electricity. Three new coal-fired power plants are proposed here: IPP 3 near Delta, Nevco's merchant plant near Sigurd in the Sevier Valley and PacifiCorp's Hunter 4 in Huntington.

Reid's opposition illustrates a growing national trend. By this past May, power companies had announced plans to build up to 150 new coal-fired plants to add to the 645 coal plants producing about 55 percent of the country's electricity, the Wall Street Journal reported Wednesday. But the same concerns that Reid expressed have stalled or canceled many of them. According to the Journal, fading coal prospects led to Citibank's downgrade of coal company stocks, saying "prophecies of a new wave of coal-fired generation have vaporized."

Along with air-quality concerns, activists have linked the Silver State's power plant proposals to the Southern Nevada Water Authority's groundwater withdrawal from Spring Valley, Nev., a project that could affect ranchers in Utah's Snake Valley. Coal-fired power plants use tremendous amounts of water in their operations.

In a Thursday interview with The Associated Press, Reid said supporting the eastern Nevada projects would be politically advantageous. But "I can't do it. My conscience wouldn't let me," he said.

"All these power moguls want to do is to steal our air and water," Reid said in the AP interview. The power plants might be good for economic development in rural areas, "but this isn't good for Nevada," he said.