ASHINGTON, Jan. 26 - The Congressional battle over how to reduce air pollution from power plants began anew on Wednesday with consideration of the approach most favored by the White House.

But after three hours of testimony on that initiative, the Clear Skies Act of 2005, it was obvious that nothing had diminished the concerns that scuttled an earlier version of the legislation. Indeed, one co-sponsor conceded that without major compromises, the new bill was most likely doomed.

"If everybody's hunkered down, it's the same old story we've had for the last five or six years," said the lawmaker, Senator George V. Voinovich, Republican of Ohio. "Then it's goodbye."

The effort to pass the Clear Skies Act as an amendment to the Clean Air Act is being led by Senator James M. Inhofe of Oklahoma, chairman of the Environment and Public Works Committee, who reintroduced it with Mr. Voinovich on Monday.

The measure sets limits on three major pollutants that affect human health - sulfur dioxide, nitrogen oxides and, for the first time, mercury - but not on carbon dioxide, a heat-trapping gas that scientists say contributes to global warming.

Mr. Inhofe, mindful of the costly technology needed for industry to control emissions, has made no secret of his opposition to carbon dioxide caps, calling global warming "the greatest hoax ever perpetrated on the American people."

But supporters of capping carbon dioxide include at least six Republicans, and it was a measure of the intense division between the bill's backers and opponents that Mr. Voinovich concluded Wednesday's hearing by declaring, "If climate change is part of this legislation, it's going nowhere."

The testimony, before the Subcommittee on Clean Air, Climate Change and Nuclear Safety, which Mr. Voinovich leads, reflected the usual fault lines on the Clear Skies Act.

Industry groups and government officials spoke in favor of it, saying it was the best way to protect jobs, local economies and investments in new power plants while aiming for the 70 percent reductions in three major pollutants that the bill promises by 2018.

Environmental groups and state environmental regulators opposed it, arguing that Congress should use the opportunity to pass a more muscular bill. They said alternative approaches would not only set carbon dioxide caps but also reduce those three other pollutants at a faster pace and, contrary to the Clear Skies measure, leave in place parts of the Clean Air Act like "new source review," which requires operators to add new pollution controls when plants are upgraded.

Two alternatives to the Clear Skies Act are measures co-sponsored by Republicans, including Senator Lincoln D. Chafee of Rhode Island, a member of Mr. Inhofe's committee. Mr. Chafee's preference for his own bill would appear to leave the committee evenly split, 9 to 9, on the Clear Skies legislation, a particular threat to its prospects.

The Clear Skies Act is modeled on a program adopted in 1990 to reduce acid rain. In setting limits on the emission of sulfur dioxide, nitrogen oxides and mercury, it would allow companies whose emissions are lower than the caps to sell credits to those whose emissions are above it. It seeks to bring most of the nation's counties into compliance with air quality standards by 2018; currently, almost half of all Americans live in areas not in compliance.

The bills competing with it propose tighter caps in a shorter period. They also retain new source review, which supporters of the Clear Skies Act contend threatens plant owners' economic stability because of expensive litigation. Since 1999, the government has brought 15 lawsuits against power companies on grounds of new-source-review violations.

In testimony Wednesday, Ron Harper, chief executive and general manager of the Basin Electric Power Cooperative, an electrical generation cooperative in nine states, said companies like his were thwarted in planning efforts because of the uncertainties of litigation.

But Conrad G. Schneider of the Clean Air Task Force, who testified on behalf of four environmental groups, said the other bills - or even doing nothing - would be preferable to Clear Skies, which he said would weaken the Clean Air Act, causing unnecessary death and illness.

"Our first principle is do no harm," Mr. Schneider said. "Clear Skies guarantees that we will never solve these problems. It offers only half measures with pollution reductions that are too little, too late."