Prodded by President Bush's call for action on his "Clear Skies" initiative, Senate Republican leaders changed course yesterday and vowed to vote this year on legislation to sharply reduce power plant emissions of health-threatening pollutants.

The administration has opposed mandatory reductions of carbon dioxide and other greenhouse gas emissions, but Bush was adamant Tuesday night in calling on Congress to pass legislation that he said would slash three other major pollutants by 70 percent over the next 15 years. Senate Environment and Public Works Committee Chairman James M. Inhofe (R-Okla.) -- who recently said there wouldn't be enough time this year to take up Bush's premier environmental initiative -- told reporters yesterday, "I think there will be a markup on [Bush's air pollution plan] this year."

"We want to look at it and look at it carefully," said Inhofe, who will formally introduce the legislation in the next couple of weeks. "It's a great start, and it is the most ambitious [plan] of any president in terms of reducing pollutants."

Sen. George V. Voinovich (R-Ohio), chairman of a subcommittee with chief responsibility for clean air issues, also signaled a willingness to negotiate a compromise with Democrats and environmental leaders. They have raised concerns about greenhouse gas emissions, as well as the pollutants nitrogen oxide, sulfur dioxide and mercury, which the Bush plan targets for cuts.

Without a compromise on greenhouse gases, "we probably can't get to nox, sox and mercury," Voinovich said in a phonetic reference to the pollutants' chemical symbols. The former governor has been a champion of the utility and coal industries, which oppose limits on carbon emissions.

The abrupt turnabout in Senate Republican tactics caught many by surprise. Capitol Hill insiders have speculated for months that Congress would sidestep controversial clean-air legislation this year, while the administration pursued regulatory changes to weaken enforcement of existing industrial clean-air programs.

The turnabout also raised the prospects for a bitter fight this year between the administration and Democratic lawmakers, independent Sen. James M. Jeffords (Vt.) and environmental leaders who contend that Clear Skies would permit more lethal power plant pollution for a longer period of time than if the Environmental Protection Agency fully enforced existing Clean Air Act programs. Some environmentalists say Bush's plan would achieve far less in reductions over the next two decades than the administration asserts.

"The president's plan actually rolls back existing air pollution laws, allowing any power plant in any community to increase emissions without safeguards to protect the public," said David G. Hawkins, director of the Natural Resources Defense Council's Climate Center. "We need tough enforcement of the Clean Air Act to protect. Instead, the White House is siding with power plant owners and other big polluters."

The president's proposal, announced a year ago, would create a mandatory emissions reduction program, establishing a national cap and credit-trading program for power plant emissions of sulfur dioxide, nitrogen oxide and mercury. Scientists blame those three pollutants for smog, haze and acid rain in urban areas and national parks; tens of thousands of premature deaths; and a host of public health problems, including heart disease, asthma and other upper respiratory illnesses.

Bush announced his plan as he outlined proposals for encouraging voluntary reductions in greenhouse gases rather than mandatory cuts called for under the Kyoto Protocol, an international agreement Bush disavowed shortly after taking office. Sens. John McCain (R-Ariz.) and Joseph I. Lieberman (D-Conn.) this month introduced a bill to require all U.S. power plants and industries to set mandatory targets for reductions of greenhouse gas emissions, an approach the White House opposes.

Some House leaders have said they will take up the Clear Skies legislation this year.

Yesterday, Environmental Protection Agency Administrator Christine Todd Whitman, Interior Secretary Gale A. Norton and Agriculture Secretary Ann M. Veneman said at a news conference that Clear Skies would have far-reaching benefits in urban and rural areas and would cut down haze in Great Smoky Mountains National Park and other parks across the country.

"We're just going to come at that very strongly," Whitman said.

"Despite the fact that oftentimes we get criticized, we think we have a very strong environmental record," Veneman said.

Meanwhile, the Adirondack Council, an upstate New York environmental organization, said Congress has "an historic opportunity" with the president's plan to stop acid rain, smog and haze from harming the environment and public health.