ASHINGTON, Jan. 8 — At a packed committee hearing, Senators John McCain, Republican of Arizona, and Joseph I. Lieberman, Democrat of Connecticut, joined forces today to challenge the Bush administration on global warming.

The two former — and possibly future — political rivals to President Bush and each other offered their bill to reduce the emissions of heat-trapping gases, saying the administration was stuck in neutral on that crucial environmental matter.

"The United States is responsible for 25 percent of the worldwide greenhouse gas emissions," Mr. McCain, chairman of the Science, Commerce and Transportation Committee, said as he opened the hearing. "It is time for the United States government to do its part to address this global problem, and a discussion of mandatory reductions is the form of leadership that is required."

That assertion was a direct challenge to Mr. Bush, whose approach relies on voluntary reductions, and it guaranteed that the McCain-Lieberman bill would face stiff opposition, particularly after next week, when Mr. Lieberman is to announce that he is seeking the Democratic nomination for president next year.

The bill, forged with advice from industry and environmental groups, would require that by 2010 industries cut emissions of carbon dioxide to 2000 levels and by 2016 to 1990 levels.

It would create a "cap and trade" system under which companies that failed to meet the goals could buy "credits" from companies that exceeded them, an approach used to reduce acid rain. The program would apply to electric utilities, industrial plants, transportation and large commercial enterprises, which were responsible for 85 percent of emissions in 2000 in the United States.

The administration declined to join the Kyoto treaty on climate change, even though last year Mr. Bush accepted findings by a panel of American experts that human activity had caused most of the global warming in recent decades.

Mr. Bush set a policy that until 2012 would rely on voluntary measures by industries to slow growth in emissions of carbon dioxide and other heat-trapping gases. He said more research was needed to clarify the potential environmental risks of warming before taking stronger measures, although White House officials said recently that they might speed their timetable in seeking compliance.

Today, the White House was unenthusiastic about the McCain-Lieberman approach.

A spokesman, Scott McClellan, said: "We're already making great progress on the president's common-sense plan to significantly reduce the growth in greenhouse-gas emissions and working with Congress to pass the most significant reduction in power-plant emissions ever."

Senator James M. Inhofe, the Oklahoma Republican who is chairman of the Environment and Public Works Committee, said the causes of global warming were still open to question. He indicated that he had a fundamental difference with the McCain-Lieberman approach by noting that the Congressional Budget Office had found that cap-and-trade programs amounted to "an energy tax on consumers."

Many people at the hearing rejected the idea that the White House was doing something serious about global warming and criticized the administration for saying it needed to do more research.

Mr. Lieberman, the first witness, said the administration's approach would "allow greenhouse-gas emissions to keep increasing indefinitely, presenting this country and the world with a bigger and bigger environmental crisis to tackle down the road," hurting the economy and America's stature in the world.

Of his bill, he said, "we do less than is explicitly called for under the Kyoto agreement, but we sure do a lot more than nothing."

Speaking for the administration at the hearing was James R. Mahoney, assistant commerce secretary and deputy administrator of the National Oceanic and Atmospheric Administration.

Dr. Mahoney said, "We do have evidence of global change," but he added, "There are substantial uncertainties about causes, and because of that uncertainty about causes, there's also substantial uncertainty about mitigation methods that might be effective."

He said the administration was spending $1.7 billion on research. Dr. Mahoney also announced that the White House would be the host of an international Earth Observation Summit this summer to discuss climate change.

And he warned that the McCain-Lieberman approach of mandatory reductions in gases would lead to "years of litigation."

Mr. McCain said a voluntary approach did not "meet the urgency" of the threat from global warming.

Senator Ron Wyden, Democrat of Oregon, was more dismissive.

"I don't think these workshops are going to cut it," Mr. Wyden told Dr. Mahoney, asking him what event, "short of flooding the National Mall," would persuade the administration that global warming was a serious problem.

Dr. Mahoney said that was above his pay grade, but offered that the administration had to worry about the "economic dislocation" caused by any mandatory program.

The hearing was the first by this committee in the new Congress, a sign that Mr. McCain wanted to make a statement. That statement, his opponents said, was that he and Mr. Lieberman were posturing.

"This was a political statement," said Frank Maisano, representing the Business Coalition on Global Warming, who predicted widespread opposition to the bill. "It's so broad, and it covers so many people and it would be so complex and complicated. It's too burdensome for anyone to fathom."

Representatives of environmental groups were thrilled with the McCain-Lieberman approach.

"It's a very big event that McCain and Lieberman would open up with this," David Doniger of the Natural Resources Defense Council said.

n an aggressive effort to show that President Bush's voluntary climate strategy can work, senior administration officials are traveling the country collecting written promises from industries to curb emissions of gases linked to global warming.

White House officials, insisting on concrete commitments measured in tons of gases, have rejected written offers from some industry groups to take nonspecific actions, several industry officials said. The administration and industry leaders plan to unveil a broad array of pledges at the White House on Feb. 6.

This is the administration's latest and most intensive effort to demonstrate that voluntarily controlling emissions can make mandatory reductions unnecessary. Mr. Bush has said such reductions will harm the economy. The effort has no teeth, officials and company representatives say, other than the growing realization in industry that without measurable success from voluntary reductions, it will become ever harder in coming years to stave off legislation requiring companies to act. Senators of both parties introduced such legislation in Congress this month, and states are acting on their own as well.

The administration's intent, once all the industries' commitments are tallied, is to meet Mr. Bush's stated goal: an 18 percent reduction, by 2012, in emissions of greenhouse gases for each unit of gross domestic product. Overall emissions would continue to grow, but more slowly.

Some company officials and other opponents of regulation have criticized the administration's effort as a mandatory program disguised as a voluntary one.

"This is meant to give the impression that the administration is doing something to control CO2 emissions," said Myron Ebell, a climate policy expert at the Competitive Enterprise Institute, which promotes free markets and limited government. "The danger is that they could easily get pushed from that position into actually regulating emissions, which would be very expensive, pointless."

At the same time, many scientists, environmental groups and political foes of Mr. Bush have said his target is so modest that no matter what industries do to achieve it, it will not help stem climate change. Most other industrialized countries have chosen to pursue binding reductions in emissions through the Kyoto Protocol, the climate treaty Mr. Bush rejected shortly after taking office.

"Over a decade ago, the United States committed to voluntary greenhouse gas reductions, and emissions have continued to rise," said Elizabeth Cook, an expert on corporate environmental policies at the World Resources Institute.

Citing an expanding body of research pointing to rising concentrations of carbon dioxide and other greenhouse gases as a cause of global warming, she and other critics said more action was needed.

White House officials said the new effort was just the beginning of a protracted campaign for voluntary reductions. "We're not declaring victory here and going home," an administration official said. "It'll be an ongoing thing from here."

Many big companies, expecting that regulation of greenhouse gases is inevitable, have already moved independently to set up voluntary caps and trading schemes in which companies that aggressively cut their emissions acquire pollution credits they can sell to other companies. The list of such companies includes most of the country's biggest energy, mineral and industrial concerns, including DuPont, Motorola , Waste Management Inc. and American Electric Power, a Midwestern utility that is the largest emitter of greenhouse gases in the Western Hemisphere.

The newest effort began on Thursday, with the start of the Chicago Climate Exchange, under which big manufacturers and energy companies agreed to cut emissions and trade credits with one another.

As they considered the administration's initiative, industries at first resisted committing themselves to specific targets.

The American Petroleum Institute, the oil industry's principal trade group, initially offered the White House a proposal for efforts on emissions, but without a specific timetable or targets. It cited the difficulty of getting all its members to agree on a single plan — and of measuring emissions from every facet of far-flung operations.

That was rejected, but after several rounds of discussions with the administration, the institute — like other industry groups — agreed to emissions changes that would mesh with Mr. Bush's 2012 goal.

"Oil, gas and other industries have all had significant discussions in trying to achieve the types of commitments the administration is desiring," said Robert L. Greco III, a senior manager at the institute. "Industry is committed to supporting this type of approach and is willing to step up to further the objective of the president's program."

Trade groups for companies pumping oil, mining coal, making cars, synthesizing plastics, smelting metals and manufacturing microchips have been recruited and have scrambled to settle on various targets for reducing or in some cases eliminating emissions.

These include some of the most influential voices for industry in Washington, the American Chemistry Council, National Mining Association, the Alliance of Automobile Manufacturers and the Edison Electric Institute, which represents power-plant owners.

Talks are still under way, and agreements could change, but some details are starting to emerge.

Under the program, magnesium producers have agreed to eliminate releases of a potent heat-trapping greenhouse gas, sulfur hexafluoride, by 2010. The gas is very rare, but each molecule has 23,600 times as much heat-trapping potential as a molecule of carbon dioxide.

Chip makers have said that by 2010 they will cut emissions of perfluorocarbons, another potent warming gas, 10 percent below 1995 levels.

Among other actions, all the major oil companies have agreed to scour pipelines and oil fields for leaking methane, another powerful heat-trapping gas. Coal companies have promised to expand efforts to capture methane and other greenhouse gases escaping from mines.

Individual companies are being asked to set more general goals.

Under a simultaneous initiative, also to begin on Feb. 6, the Business Roundtable, which represents 140 of the country's biggest companies, is working with the White House to obtain commitments from its members to start assessing their activities and considering ways to reduce their impact on climate.

Although that effort is theoretically voluntary, the Business Roundtable has already promised to deliver 100 percent of its members.

Some industry officials have quietly objected to the heavy pressure to sign on.

On Jan. 8, James L. Connaughton, chairman of the White House Council on Environmental Quality, addressed a private gathering of leaders of electric utilities at the Ritz Carlton in Naples, Fla. Several executives who were there said his insistence on substantive commitments prompted some of them to label the effort the "mandatory voluntary climate program."

The administration's push has intensified as criticisms of its cautious climate policies have increased, and more aggressive alternatives have been proposed.

On the day Mr. Connaughton spoke in Florida, Senator John McCain, Republican of Arizona, and Senator Joseph I. Lieberman, Democrat of Connecticut, unveiled a bill that would require restrictions on emissions. California and New York are moving toward restricting greenhouse gases from vehicles.

Administration officials acknowledge that they are trying to tread a fine line. They do not want to alienate voters in states like West Virginia, where the economy revolves around coal, a major source of carbon dioxide, but they do want to appease moderates, particularly women, for whom global warming is a growing concern.

But in seeking that path, many experts and lobbyists for different factions said, the administration could end up satisfying no one and doing little to solve the problem.

Many people involved in the White House effort, including government officials and executives from industries, say it is unlikely to lead to improvements much beyond those already taking place as the economy shifts from old-style manufacturing and businesses grow less wasteful.

And the effort, aimed mainly at manufacturing, encompasses only a small portion of America's greenhouse-gas emissions.

For example, while the auto industry is agreeing to curb gases from its assembly lines, it has not been asked — nor has it promised — to reduce gases from the tailpipes of the cars and trucks it builds.

Nevertheless, Ms. Cook, at the World Resources Institute, said there was some value in finally pushing a broad array of industries to start looking for ways to reduce their impact on climate. Once they have committed to change, she said, it will be hard for them to reverse course.