When Barack Obama was elected president, he was heralded as a possible savior for climate-treaty talks that had dragged on for years while George W. Bush rejected limits on US greenhouse-gas emissions.

“America is back” at the United Nations negotiating table, Democratic Senator John Kerry declared after the November election. Danish climate minister Connie Hedegaard said US emissions policy moved forward 35 years overnight.

Instead, Obama may send empty-handed envoys in December to the table in Copenhagen where 192 countries will try to assign emissions reductions because Congress has given him no mandate. With the European Union, Japan and Australia ready to pledge cuts of more than 20 percent only if other nations follow suit, the stage is set for promises to collapse.

“How can we expect other major players to move their position until they know that in the end the US is also going to deliver?” Hedegaard, chairman of the UN talks running from December 7 to 18, said in an interview.

The possible domino effect, along with a continuing split between the US and China, erode chances for a strong treaty, negotiators and political scientists say.

“It is unlikely that an agreement which would be meaningful is going to be finalized” in the Danish capital, Robert Stavins, director of the Harvard Environmental Economics Program in Cambridge, Massachusetts, said in an interview.

When Obama picks up his Nobel Peace Prize in Oslo in December, he’ll be an hour’s flight from where more than 10,000 envoys, UN officials and lobbyists will be meeting to conclude an agreement on slowing climate change, a challenge the president has said the US will “lead the world” in tackling.

Obama hasn’t decided whether to make an appearance, administration officials said. Environmentalists say he’s a likely no-show because stalled climate bills in Congress mean the US may have little to offer, threatening to unravel prospects for a global deal.

“If this were a play getting ready to come to Broadway, we would say: ‘Well, we aren’t sure of the financial backing or the orchestra and, guess what, the lead star says he might not sing,’” said Peter Goldmark, director of the climate and air program at New York-based Environmental Defense.

There are other stumbling blocks beyond the US.

Industrialized nations are still split with developing countries such as China, the largest greenhouse-gas producer, over transferring clean-energy technologies to poorer nations and over climate-adaptation aid.

Yet the heart of a climate deal is to get nations to slash heat-trapping gases such as carbon dioxide that scientists blame for global warming. That was the UN’s principle conclusion in a 2007 declaration made in Bali, Indonesia, that proposed a “road map” for forging a new treaty in Copenhagen two years later.

Obama has said he’ll push to cut US emissions back to 1990 levels by 2020. Moving on his lead, the US House passed legislation in June to create a cap-and-trade system that would limit gas emissions and create a market in pollution permits.

On Tuesday, Senate committee hearings were set to begin in Washington on a comparable measure. Congressional leaders, embroiled for months in the debate over health-care legislation, made no promises that the full Senate will take up the climate bill before 2010.

The moderate Democrats and Republicans “we are counting on” to back climate-change legislation are preoccupied with overhauling the US health-care system, said Bob Simon, chief of staff of the Senate Energy Committee.

Having no Congressional mandate will make it “extraordinarily difficult” to commit to a target in treaty talks, US lead negotiator Jonathan Pershing said early this month at the last negotiating round in Bangkok.

If the US, the second-largest emitter, could deliver, other countries might firm up their commitments, Brice Lalonde, France’s lead negotiator, said in a telephone interview. He pointed to carbon-reduction pledges by many developed nations that are conditional on a deal being reached in Denmark.

“Look at all the commitments which are conditioned by ‘if,’” Lalonde said. “Take out all these ifs and we’ve got an agreement.”

Only if other nations take similar steps will Japan and Australia cut their emissions 25 percent by 2020, they said. Commitments by New Zealand and Switzerland are also contingent on a wider deal. And the 27-nation European Union has pledged a 20-percent greenhouse-gas cut by 2020 from 1990 levels and will ramp that up to 30 percent if comparable action is taken by other developed nations.

Developing nations, whose emissions are growing faster than in richer countries, might be motivated to talk about when they can “peak” their greenhouse gases before starting to cut them, if they got a clear stance from the US, Lalonde said.

India, Brazil and Mexico also may be looking to the world’s biggest economy for action before fully committing, said Jake Schmidt, international climate policy director for the Natural Resources Defense Council, a New York-based environmental group.

The US climate position avoids giving too much economic leeway to China, a growing industrial competitor that rejects binding emissions targets. China says that as a developing nation the priority is to pull its people out of poverty. The two nations release 40 percent of global emissions.

Even so, Chinese President Hu Jintao has said that his country will cut emissions in proportion to economic growth by a “notable” margin by 2020.

“It’s fair to say that the Chinese are holding back on putting forth a number in terms of their greenhouse gas intensity target until they have greater clarity on what the US is going to do,” said Schmidt. “If the US put forward a number and gave a clear signal of what it would do, I think China would follow minutes after that.”

American negotiators say they don’t want to bring a deal back home to the Senate, the only US body that can ratify a treaty, and get it rejected. Former President Bill Clinton’s UN envoy Peter Burleigh signed the Kyoto Protocol accord in 1998. Neither Bush nor Clinton sent the treaty to the Senate. The 100-member chamber said at the time it would reject an accord that didn’t make requirements of developing nations such as China.

“We don’t want to repeat the Kyoto experience of having a number where there’s nothing behind it,” Obama’s climate envoy, Todd Stern, said.

By avoiding Kyoto, the world’s biggest energy consumer was allowed to increase emissions by about 16 percent from 1990 through 2007, UN data show. Kyoto demanded a 7 percent reduction from the same base year to the 2008-2012 measurement period.

“Are we going to be forever hostage to the US Congress?” asked Bernarditas Muller, a negotiator for the Philippines who helps coordinate the G77 group, an alliance of 130 developing nations.

Just as in the 1990s, Congress is concerned over losing jobs to low-cost economies with no emission rules, and members are considering setting duties to be paid on their imports.

“The negotiations need to move forward on what we do with leveling the playing field on manufacturing so that a country that acts responsibly on CO2 emissions doesn’t lose jobs to those countries that don’t,” said Senator Sherrod Brown, a Democrat from Ohio.

Uncertainty over the US position “is causing countries to hold back what they might do as well,” said the NRDC’s Schmidt, who’s tracked international climate negotiations since 2000. “It’s having a big ripple-on effect for the rest of the negotiations.”

Obama and many world leaders probably will send underlings to the Danish capital instead of traveling themselves as the chance of countries breaking a deadlock are slim, said Eileen Claussen, head of the Pew Center on Global Climate Change in Arlington, Virginia.

“There is a lot of pressure on the president to go,” Claussen said. “But it’s hard to imagine heads of state going to Copenhagen and ending up with a political declaration that says: ‘It’s really hard and we are going to take another six months to do it.’”