China has been hesitant to commit to a deal where it would be open to scrutiny by the international community. Leaders from the country have balked at measures to allow international monitors to verify emissions cuts that might be agreed to in the deal.
In the last 24 hours, officials say China had softened its opposition to verification of emissions reductions. Clinton announced Thursday that the United States will contribute to a $100 billion fund by 2020 to help poor countries cope with climate change, but that financial help is conditional on China relenting on verification, a move that could exert pressure on China.
But China and the United States appeared to be divided on the issue of verification.
The notion that the success of the climate agreements may hinge on the United States and China frustrated leaders of some developing countries.
Speaking as the voice of the developing world, Brazilian President Lula da Silva gave a passionate speech where he scolded the developed world for not negotiating on climate change in good faith with poorer nations. He also said this conference is not about climate change, but about economic opportunities for the developing world.
With an array of challenges the president has to confront at home -- from health care to unemployment to the economy -- the president is putting his prestige at stake, both at home and abroad, to fly to Copenhagen and seal a deal on an issue that lacks the support of China, one of the biggest polluters in the world.
The U.S. president usually goes on trips like this when an agreement has already been worked out, but the White House said it is not a surprise that Obama went. His job as a leader, sources say, is to lead and that's what he wants to do in Copenhagen.
Obama, whose Nobel Peace Prize win was partially attributed to his efforts on climate change, does not only face the skepticism of an international audience, but even his supporters back home. If he didn't go to Copenhagen, he would upset his liberal base.
Addressing world leaders earlier today, Obama stressed the urgency of coming together on a deal and raised the deflating prospect of a deal.
"While the reality of climate change is not in doubt, I have to be honest, I think our ability to take collective action is in doubt right now and it hangs in the balance," Obama said at a plenary session at the United Nations summit.
"This is not a perfect agreement, and no country would get everything that it wants," the president said. "But here is the bottom line: we can embrace this accord, take a substantial step forward, and continue to refine it and build upon its foundation. We can do that, and everyone who is in this room will be a part of an historic endeavor, one that makes life better for our children and grandchildren."
Obama outlined three key elements that must be in the final agreement. First, all major economies must commit to reducing their emissions. Second, there must be a transparent review to ensure nations are keeping their commitments to reduce emissions, and thirdly, there needs to be financing to help developing nations adapt to these new standards.
What world leaders do agree on are the stakes. With unprecedented urgency, a chorus of leaders have delivered the same warning -- the planet is already damaged by climate change and countries need to step up to confront this challenge.
"We have a moral obligation towards future generations," said Prime Minister of Denmark Lars Lokke Rasmussen.
UK Prime Minister Gordon Brown described the stake in apocalyptic terms Thursday.
"Without common action, extreme temperatures will create a new generation of poor with climate change refugees driven from their homes by drought, climate change evacuees fleeing the threat of drowning, the climate change hungry desperate for lack of food," Brown said. "Hurricanes, floods, typhoons and droughts that were once all regarded as the acts of an invisible god are now revealed to be also the visible acts of man."
ABC News' Karen Travers, Kirit Radia and Viviana Hurtado contributed to this report.