With the prospect of a comprehensive global warming deal melting away, President Obama forged ahead anyway with his trip to Copenhagen on Friday hoping to score a substantial agreement that can revive momentum for one of his top priorities that has stalled in Congress.

"We're not going there just to get an agreement for the sake of something that's called an agreement," White House spokesman Robert Gibbs told Fox News on Thursday. "We want something that works for both the international community but also something that works for the United States."

Asked if Obama feared returning to the U.S. "empty-handed," Gibbs said, "Coming back with an empty agreement would be far worse than coming back empty-handed."

But repeatedly citing the "incredibly complex" negotiations stalling a long-sought climate change deal here, senior Obama officials, speaking hours before the president was to leave, said they could not predict a breakthrough but said they hoped the U.S. "would have an impact" on last-ditch efforts to salvage some form of compromise.

In what may be a concerted effort to play down expectations, two senior Obama officials said the summit would have to wrestle with global warming issues long after all 130 heads of state depart the Danish capital. They appeared resigned, on the eve of the talks' conclusion, to accepting vague pledges for cuts in greenhouse gas emissions rather than the binding commitment sought when talks began here two weeks ago.

"It's a challenging set of negotiations," one top official said. "This is an on-going process. This is the 15th year of this process."

Obama's trip comes as the U.S. and China took steps Thursday toward a broad agreement that could be sealed by Obama and Premier Wen Jiabao when they arrive at the flagging U.N. climate talks.

But it remains to be seen what concessions Obama will have to make for the U.S. to seal the deal on emissions actions at the 193-nation conference.

Secretary of State Hillary Clinton already announced the U.S. would press to raise $100 billion a year worldwide by 2020 to help poorer nations cope with global warming, and China offered to open its books on efforts to cut emissions.

White House officials said they hoped the final result would be a so-called operation agreement -- weaker than a binding set of greenhouse gas limits with measurable target dates but stronger than a political commitment with no underlying action plan.

But there was no guarantee of this outcome due to differences between the U.S. and China over pollution limits and the ability of outside nations to verify compliance - an issue covered by the umbrella term "transparency."

Republican lawmakers were not encouraged by the developments.

"Clearly, the things that are happening in Copenhagen run counter to what Americans want this administration to be focused on," Rep. Steve Scalise, R-La., told FoxNews.com. "Instead you have the secretary of state saying the U.S. will give another $100 billion of money we don't have to countries who don't like us."

But a senior administration official said Thursday that Obama will not announce more financial commitments and believes his administration has demonstrated that the U.S. is willing to take all steps possible to fight global warming.

Still, the U.S. is under renewed pressure to improve its pledge of greenhouse-gas emission cutbacks -- by about 17 percent by 2020, compared with 2005 figures. That's only a 3 percent to 4 percent reduction from 1990, the benchmark year for the Kyoto countries and the basis for the European Union's pledge to cut emissions by at least 20 percent by 2020.

"I have to be honest, an offer by the United States to cut only 4 percent from 1990 levels is not ambitious enough," German Chancellor Angela Merkel told lawmakers in Berlin before arriving in Copenhagen.

But Obama can't make a stronger offer without the support of the Senate, where climate change legislation is stuck and won't be taken up again until next year.

"What we are seeing is the can being kicked down the road to the next meeting of parties" in Mexico, said Pat Michaels, a senior fellow in environmental studies at the libertarian Cato Institute. Michaels said he didn't believe Obama's appearance at the summit would accomplish much, making it "an injudicious use of fossil fuel."

The biggest obstacle for the U.S. has been China as a diplomatic duel between Washington and Beijing has marked the two weeks of climate talks.

Gibbs expressed hope that China would not abandon the remaining talks, undermining prospects for any credible global warming deal.

"We hope the Chinese will state and be part of finding a solution," Gibbs said. "If the Chinese are unwilling to be able to prove to the world that they can live up to an agreement that they make, then I think it calls into question whether or not you truly have an agreement. The elements of getting an agreement are there if countries like China will make some common sense agreements about transparency."

Gibbs laid to rest speculation that Obama would cancel his trip as the summit ground to a halt Wednesday over a dispute between rich and poor nations. There is "no change in our plans," he said.

Global warming skeptics suggested Obama had no choice but to attend, because he has staked the first year of his presidency on the issue.

"They're going to try and make the best of a lose-lose proposition," said Pat Creighton, a spokesman for the conservative Institute for Energy Research.

Scalise, a member of the Energy committee, was at a loss to explain why Obama would stick to his planned trip amid so much uncertainty and lower expectations.

"I think he's still trying to find some way to justify his Nobel Peace Prize, but it's coming at the expense of the U.S. economy and is based on corrupt science," he said, referring to the "Climate-gate scandal" in which stolen e-mails at a prominent university showed researchers manipulated data to make their case for global warming.

Fox News' Major Garrett contributed to this report.