Supporters of limits on greenhouse gases are betting that the road to U.S. climate-change legislation runs through China.

This year, China is expected to surpass the U.S. as the leading producer of greenhouse gases, and one reason the Bush administration has declined to enact emissions limits is its concern about leaving China unchecked.

So lawmakers, environmentalists and business leaders are looking at ways to engage China in the battle against climate change — or pay the price for doing nothing. Proposed strategies range from providing financing and technology transfers to imposing special carbon-based import tariffs or changing supply contracts.

Some are looking toward measures that are within control of Congress and could be part of a climate-control package. "There is a lot of discussion up on the Hill about whether within the context of U.S. legislation there would be language that would put China on the hook for action as well," said Joanna Lewis, a senior international fellow at the Pew Center on Global Climate Change.

Kathleen A. McGinty, secretary of Pennsylvania's Department of Environmental Protection and former adviser to Vice President Al Gore, thinks import tariffs are one option. She argues that it would not violate World Trade Organization rules, which she says allow tariffs "when public health is at stake."

Michael G. Morris, chief executive of American Electric Power, recently made a similar suggestion in testimony before the House Energy and Commerce Committee.

"If other countries refuse to reduce emissions but seek to continue to sell their goods in the U.S., our proposal would implement an appropriate measure to equalize the conditions of global trade," Morris said. He said such measures could include "border adjustment taxes" equal to domestic greenhouse gas costs.

Not surprisingly, the Morris proposal has the support of Edwin D. Hill, president of the International Brotherhood of Electrical Workers. Though it has an environmental rationale, it would be a protective tariff of the sort that many unions have long fought for in a losing battle against free-trade advocates.

Environmental groups have different views. "Connected to the right domestic legislation, we think that proposal is a way to solve the domestic political problem in a way that lets us move forward at home without disadvantaging American companies and workers," said David Doniger, policy director of the climate center at the Natural Resources Defense Council. "It also creates some diplomatic leverage to use to get China to take on stronger commitments."

Others see the tariffs or fees as efforts to sink climate-change legislation. "China's rapidly growing emissions are a serious issue," Phil Clapp, president of the National Environmental Trust, said. "But many diehard opponents of enforceable limits on global warming pollution who now can't hide behind the science are trying to hide behind China. Note who keeps raising the China issue: the coal industry, the oil industry, members of Congress from coal states and the auto industry. They raise threats to American competitiveness that are bogus, on the whole."

Some businesses are taking their own steps to influence Chinese behavior — and polish their own images as "green" companies. Computer maker Dell, which spends $16 billion a year on Chinese-made parts, said it would add a question about carbon emissions to the quality-performance reviews it gives its suppliers. "The total score is a big part of whether they get our business," said Dell spokesman Brian Hilton.