In the face of GOP opposition, the EPA on Monday declared greenhouse gases a danger to public health in a move that could pave the way for future regulation.
The administration also waved off concerns about the controversy surrounding leaked e-mails at a British climate research center, with the U.S. envoy to the international climate change conference in Copenhagen dismissing the flap as a "small blip."
EPA Administrator Lisa Jackson said in a written statement that the finding, which declares carbon dioxide and five other greenhouse gases a threat to public health, marks the start of a U.S. campaign to tackle greenhouse gas emissions.
"These long-overdue findings cement 2009's place in history as the year when the United States Government began addressing the challenge of greenhouse-gas pollution and seizing the opportunity of clean-energy reform," she said.
The meticulously timed announcement comes on the opening day of the Denmark conference, and could boost the administration's argument that the United States is taking action to combat global warming -- though Congress has yet to pass climate legislation.
But Republicans since last week had called on Jackson to withdraw the finding pending an investigation into whether the science behind the decision has been compromised. They raised their concerns following the leak of e-mails from the University of East Anglia's Climatic Research Unit which appeared to show scientists discussing the manipulation of climate data.
Rep. Darrell Issa, R-Calif., ranking Republican on the House Oversight and Government Reform Committee, told FoxNews.com the EPA is rushing to a decision that may not be based on sound science.
"It's certainly reckless considering the underlying science now has been very much called into question," Issa said Monday. "The inconvenient truth is not Al Gore's movie. The inconvenient truth is that people who had an agenda destroyed the facts to get an outcome."
Issa said he's not a global warming skeptic, but thinks the underlying research needs to be more closely examined to make sure billions of dollars are not wasted in the course of complying with new regulations.
While administration officials have said they would prefer Congress take action on regulating greenhouse gas emissions, Republicans fear the EPA is prepared to act unilaterally to do so, buoyed by its latest finding. And they question the timing of the announcement.
"They are finalizing the finding just in time for President Obama to travel to Copenhagen. The EPA claims its process is dictated by science, however, it's conveniently timed to push its politics," Rep. Jim Sensenbrenner, R-Wis., ranking Republican on the House Select Committee on Energy Independence and Global Warming, said in a written statement.
The EPA signaled last April that it was inclined to view heat-trapping pollution as a threat to public health and welfare and began to take public comments under a formal rulemaking. The action marked a reversal from the Bush administration, which had declined to aggressively pursue the issue.
Business groups have strongly argued against tackling global warming through the regulatory process of the Clean Air Act. Any such regulations are likely to spawn lawsuits and lengthy legal fights.
Democrats, though, claimed that the announcement Monday only strengthens the argument for government action.
"It is now clear that if we take our responsibility seriously to protect and defend our people from this threat, the Senate has a duty to act on climate change legislation," Sen. Barbara Boxer, D-Calif., said in a statement. "In light of the EPA endangerment finding, the president's appearance in Copenhagen will carry even more weight, because it shows that America is taking this issue very seriously and is moving forward."
Jonathan Pershing, U.S. deputy special envoy for climate change, said in Copenhagen that the United States is not expecting any legally binding treaty in Denmark this month, but will pursue a "political arrangement" that could lead to one in the future.
He dismissed the controversy over the leaked e-mails.
"I think it will have virtually no effect at all," he said. "The science is incredibly robust. And as we look forward, I worry much, much more about not acting urgently than what will ultimately be a small blip on the history of this process."
Asked about the remark, Issa said: "Richard Nixon said that about what Deep Throat had outed about the break-in."
The Associated Press contributed to this report.