With the fate of President Bush's Clear Skies proposal looking increasingly hazy on Capitol Hill, administration officials are preparing to make similar changes to air pollution rules for power plants, which are among the United States' biggest generators of smog, acid rain and mercury.

For the third time in three weeks, Senate Republicans on Thursday postponed a pivotal committee hearing on the White House-backed Clear Skies legislation, the most ambitious overhaul of the Clean Air Act in 15 years, because they lacked the votes to pass it.

But if Clear Skies stalls, Bush administration officials are now poised to reshape power plant regulations through the rule making process later this month in ways that mimic the legislation.

In the offing are new limits on power plant emissions of mercury that have been widely criticized as too lenient.

On the other hand, the administration's proposed changes include market-based incentives to reduce smog and acid rain which critics of Clear Skies support.

The administrative changes would not entirely accomplish the administration's objectives to ease environmental restrictions on electricity generation, particularly on coal-fired power plants.

Without Congressional approval, for example, the administration cannot eliminate requirements in the Clean Air Act that are about to compel power plants to invest billions of dollars in pollution control technologies in coming decades.

But the changes may represent all that is politically possible, given bi-partisan objections in the Senate.

Clear Skies would replace the complicated patchwork of Clean Air Act regulations that currently restrict power plant pollution with a simpler rule that is favored by coal-burning utilities. It aims to reduce emissions of principal power plant pollutants through what is known as a cap and trade system.

The Clear Skies rule would set a national ceiling on the emissions, and power plants that cut more than their required share would obtain surplus "pollution credits" that they could sell to power plants that exceeded their quotas, giving them a financial incentive to clean the air.

Bush administration officials assert that Clear Skies would cut pollution faster and with far less regulatory red tape than existing Clean Air Act rules while still maintaining a vital place for coal in the nation's energy mix. They argue that it is economically important to ensure that the next generation of clean air rules does not force utilities to abandon coal, the nation's cheapest and most abundant source of energy. Clear Skies seeks to cut power plant emissions of three major pollutants by 70 percent over the next 11 years.

Such cuts would restore clean air to most of the more than 150 million Americans who live in counties that exceed federal health requirements for smog or particle pollution, though they would not solve California's extraordinary air quality problems.

"No one here thinks the Clean Air Act could get these types of reductions," said Jeffrey Holmstead, the Environmental Protection Agency's top air quality official.

State air pollution regulators and some former EPA officials strongly disagree, arguing that the nation's air would be cleaned up faster if the Bush administration simply enforced existing rules instead of tinkering with them. The Clear Skies legislation would weaken regulations that are poised to force billions of dollars in pollution control improvements at hundreds of aging coal-fired power plants over the next 20 years, they note.

Coal mining companies, labor unions and other supporters of Clear Skies mounted a fierce lobbying push this week in hopes of breaking a deadlock in the Senate Environment and Public Works Committee that has stalled the legislation.

Senate Republicans pressured the lone GOP holdout, Lincoln Chafee of Rhode Island, while the utility industry and other backers pressed Democrat Max Baucus of Montana, who represents a region with considerable coal reserves. Republicans even made last minute changes to the bill that gave favorable status to a Montana coal mine in hopes of winning Baucus's support.

Environmental groups and the League of Women Voters made their own late lobbying pitches, arguing that Clear Skies was a dangerous rollback of clean air rules designed to benefit coal-burning power plants.

Senate Republicans failed to find the one additional vote needed to move the legislation to the Senate floor, and rescheduled the hearing for March 9 -- the second postponement of the week.