“This could be one hell of a global game changer with big reverberations here at home,” said Sen. John Kerry (D-Mass.), the lead sponsor of climate change legislation in the Senate. “The fact that the president will attend the Copenhagen talks underscores that the administration is putting its money where its mouth is, putting the president’s prestige on the line.”

And therein lies the problem for Obama. The upside potential for his visit is limited; expectations for the conference have dropped dramatically over the past few weeks, with nearly universal acknowledgment that the talks are very likely to result in a only limited political agreement, rather than in a legally binding treaty on global warming.

But the downside is significant. Obama has come home from Copenhagen empty-handed once before — when he flew in to lobby for Chicago’s pitch for the 2016 Olympics, only to watch the International Olympic Committee reject his hometown’s bid in the first round of its voting.

Another failure on the international stage could bring another round of embarrassment and set back efforts to get climate change legislation through the Senate.

The timing of the trip suggests that the White House is fully aware of the peril — and is doing what it can to minimize the risk while also avoiding a no-show that would signal to the world, and a Senate already reluctant to pass a climate bill, that the White House was not serious about one of its top agenda items.

More than 65 world leaders plan to attend the negotiations next month, including French President Nicolas Sarkozy, German Chancellor Angela Merkel and British Prime Minister Gordon Brown. Most will come for a special world-leaders session held near the end of the two-week conference. Typically, major decisions at international climate forums come during the last weekend of the conference.

Obama plans to go in the middle of the first week — timing that allows him to show an American commitment to curbing greenhouse gases but puts him at a distance if negotiators have failed to reach agreement by the end of the conference.

Some environmentalists say the timing of the visit makes it little more than symbolic.

“Attending the talks on Dec. 9th amounts to nothing more than President Obama taking a photo opportunity on his way to pick up the Nobel Peace Prize,” said Kyle Ash, climate policy adviser for Greenpeace.

Other environmentalists say Obama should return later in the talks if necessary to get a deal.

“If his presence during the latter days of the COP becomes necessary to secure the right commitments, we hope the president will be willing to return to Copenhagen with the rest of the world’s leaders during the final stages of the negotiations,” said Keya Chatterjee, director of the United States Climate Change Program at the World Wildlife Fund.

But top White House aides say they feel comfortable that negotiators can reach a substantive agreement, even if the president addresses delegates early in the negotiations.

They attribute their new confidence in the talks to recent administration talks with China and India. The two emerging economies are key to reaching a worldwide agreement. Earlier this month, Obama and Chinese president Hu Jintao released a joint statement declaring their intention to work toward accountable and transparent reductions.

But China and India, say experts, are unlikely to agree to specific reduction targets for developing nations — a sticking point that could make it hard for U.S. negotiators to sign off on a deal without facing a congressional backlash.

Manufacturing state lawmakers fear that a cap on greenhouse gases will result in a loss of jobs for their region unless China agrees to make similar reductions.

On Friday, Chinese officials are expected to announce a “carbon intensity target” to lower greenhouse gas intensity 40 to 45 percent per unit of gross domestic product. U.S. lawmakers were unimpressed when Hu first announced that goal at a United Nations speech earlier this fall, arguing that it would not significantly reduce total emissions.

Obama plans to announce that the U.S. will commit to reduce greenhouse gas emissions by 17 percent below 2005 levels by 2050. That goal echoes a proposal included in legislation that passed the House in late June.

That proposal is unlikely to satisfy European nations who have been pushing for much more serious cuts in greenhouse gases.

The European Union has vowed to reduce its emissions by 20 percent from 1990 levels (which are lower than the United States’ 2005 baseline) before 2020, and will raise the target to 30 percent in the event of an international agreement.

“As to whether Obama’s attendance will transform the results in Copenhagen, it seems unlikely,” said Frank Maisano, an energy specialist at the law firm Bracewell & Giuliani. “Progress on the major sticking points across the board remains slow.”

Congressional Democrats could pose an even larger problem back at home. Coal-, rural-, and manufacturing-state Democrats have a long list of concerns with the legislation. Several have suggested passing an energy bill that does not include the controversial cap-and-trade proposal to limit greenhouse gases.

Carol Browner, White House energy and climate adviser, said administration officials consulted with Congress before making their announcement and were encouraged by an ongoing effort by Sens. Kerry, Lindsey Graham (R-S.C.) and Joe Lieberman (I-Conn.) to craft a bipartisan bill.

But the Senate’s failure to pass a bill so far complicates the international negotiations. Foreign negotiators and ministers know too well that Congress must ratify international treaties — a lesson they learned the hard way in1997 when lawmakers failed to ratify the Kyoto climate treaty after years of international talks.

Without a clear signal that Congress will pass a bill this spring, environmentalists say the negotiations will not result in a significant agreement.

“It’s essential that President Obama communicates his personal commitment to ensuring Congress passes climate and energy legislation in early 2010,” said Alden Meyer, director of strategy and policy at the Union of Concerned Scientists. “The world needs to hear that this will be a top priority for him and the Senate once Congress completes its work on domestic health care reform.”

But if Obama commits to fast-tracking a bill through the Senate, he could face a backlash from lawmakers — including some in the president’s own party — who aren’t prepared to move the legislation.

That type of Democratic infighting has Republicans salivating.

“I suspect President Obama is making the trip to Copenhagen in order to ‘save’ the climate conference,” said Oklahoma Republican Sen. Jim Inhofe, a longtime opponent of international climate treaties and vocal global warming skeptic.

Other conservatives are already portraying the Obama trip as a celebrity photo opportunity. “It appears that he is going to mingle with other heads of state and get some good media,” said Myron Ebell, of the right-wing Competitive Enterprise Institute. “He would be better advised to stay home and pay attention to some real problems, such as 10 percent unemployment, the federal deficit and the declining value of the dollar.”