Hours after U.S. EPA declared that greenhouse gases threaten public health, a free-market advocacy group warned that it will challenge the agency's endangerment finding in federal court.

The Competitive Enterprise Institute announced its plans to sue just after the Obama administration today laid the groundwork for sweeping federal climate regulations by asserting that carbon dioxide and other greenhouse gases pose a threat to public health and welfare and that emissions from new motor vehicles contribute to the mix of those gases in the atmosphere.

"EPA is clinging for dear life to the notion that the global climate models are holding up," said CEI General Counsel Sam Kazman. "In reality, those models are about to sink under the growing weight of evidence that they are fabrications," he added, citing the series of e-mails hacked from the University of East Anglia's Climatic Research Unit, which some skeptics say shows that researchers intentionally withheld climate data and sought to stifle competing theories.

EPA Administrator Lisa Jackson deflected criticisms about the science used to underpin the agency's endangerment finding as she announced that the agency will begin to regulate greenhouse gases under the Clean Air Act.

"There is nothing in the hacked e-mails that undermines the science upon which this decision is based," Jackson said at a news conference today.

But CEI and other opponents of the administration's climate policies decried today's announcement, arguing that the administration should have stalled the finding until the e-mails have been fully investigated.

Rep. Joe Barton (R-Texas), ranking member of the House Energy and Commerce Committee, accused the administration of ignoring serious concerns about the scientists' e-mails in order to gain political leverage at international climate negotiations that kicked off today in Copenhagen and to spur congressional action.

"When the scientists whose work is the bedrock for our global warming policy use words like 'travesty' and 'trick' to describe their actions, and banter about how they'll blackball professional journals and delete evidence, it's time to slow down and consider what we're doing, not sound the charge," Barton said.

Top administration officials and climate scientists involved in the controversy have repeatedly dismissed claims that the contents of the e-mails upend the scientific consensus that man-made emissions are causing climate change.

"We know that skeptics have and will continue to try to sow doubts about the science," Jackson said. "But raising doubts -- even in the face of overwhelming evidence -- is a tactic that has been used by defenders of the status quo for years. Those tactics have only served to delay and distract from the real work ahead, namely, growing our clean energy economy and freeing ourselves from foreign oil that endangers our security and our economy."

Meanwhile, Democrats on Capitol Hill praised the administration for sending a message to Congress and international climate negotiators that EPA is moving forward with Clean Air Act regulations.

"The message to Congress is crystal clear: Get moving," said Senate Foreign Relations Chairman John Kerry (D-Mass.). "Imposed regulations by definition will not include the job protections and investment incentives we are proposing in the Senate today. Given the potential for agency regulation, those who now aim to grind the legislative process to a halt would later come running to Congress to secure the kinds of incentives we can pass today."