A year after President Barack Obama worked personally to salvage the Copenhagen climate summit, a political shift leaves the United States with far less leverage while China moves ahead.

US negotiators in the UN-led talks in Cancun, Mexico, face the tough task of persuading China and other emerging economies to agree to a binding treaty without offering any concessions that could face a backlash in Washington.

Obama's Democratic Party suffered a stinging election defeat on November 2 to the Republican Party, which has vowed to oppose a nationwide plan to restrict carbon emissions blamed for global warming.

"The United States has the leverage of any major country but its ability to promise much more is rather limited by the domestic situation," said Alden Meyer, director of strategy and policy at the Union of Concerned Scientists.

"It's pretty clear that for the next few years there is not going to be comprehensive climate legislation in the United States," he said.

"The administration has to be somewhat careful about how it plays its hand here, because it doesn't want to feed into the anti-climate rhetoric," Meyer said from Cancun.

Obama last year jetted to Copenhagen, joining other world leaders in brokering an agreement that set an aim of limiting warming to two degrees Celsius (3.6 degrees Fahrenheit) but lacked details on how to achieve it.

During the summit, Secretary of State Hillary Clinton vowed that the United States would contribute toward a fund worth 100 billion dollars a year by 2020 to help poor countries worst hit by climate change -- contingent on a "strong" accord being reached.

The United States and other developed nations have insisted that China, the world's largest carbon emitter, agree to legally binding cuts under a treaty -- which few expect to be reached in Cancun.

The dispute has been tense at times. At UN-backed talks in October, China's chief climate negotiator, Su Wei, said the United States was like a "pig looking in a mirror" and finding itself beautiful.

China has shown no signs of budging on a treaty but has ramped up investment in renewable energies such as solar and wind energy. Two recent studies found that China's investment in green technology has outpaced that of the United States.

Ailun Yang, the head of climate and energy for Greenpeace East Asia, said China was mostly acting due to domestic impulses. Its rampant use of coal is causing severe environmental problems and it fears for its energy security, with its fast-growing economy dependent on oil imports.

But Yang said that China was unwilling to play a more active role in international negotiations, despite the wake-up call in Copenhagen.

"China was as shocked as probably anybody else at the changes in the international expectations of the country. But the government just doesn't seem capable of living up to those expectations," she said.

"I think the current situation means that the United States will have less leverage in international negotiations and that China will face less pressure," she said.

For some, the negotiations are deja vu. The United States helped draft the landmark Kyoto Protocol but did not join the treaty, whose obligations expire at the end of 2012.

US lawmakers said Kyoto was unfair by making demands only of wealthy nations and not emerging economies such as China. A number of Republicans in the new Congress also question the science behind climate change.

William Antholis, managing director of the Brookings Institution and co-author of the book, "Fast Forward: Ethics and Politics in the Age of Global Warming," said it was unrealistic to expect unwieldy international conferences to draft a new treaty.

Instead, key players such as the United States, European Union, China and India should take the lead, Antholis said.

But unless the United States acts, it will be difficult to persuade smaller developed countries such as Australia, Canada and Japan to take aggressive action on climate change, he said.

"It frankly makes no sense for those countries to move if the United States doesn't move as it would put them at a competitive disadvantage against the world's biggest economy," he said.