Reporting from Washington -- The Obama administration will declare greenhouse gases a threat to public health today, sources said, marking a major step -- both practically and symbolically -- toward federal limits on the carbon dioxide emissions scientists blame for global warming.

The move by the Environmental Protection Agency is prompted by a two-year-old Supreme Court decision. It paves the way for the White House to regulate emissions from vehicles and effectively force the U.S. auto fleet to be cleaner and more efficient - a plan the administration is expected to put in place soon.

It also opens the door to broad emissions limits in all other parts of the economy, including power plants and construction sites, which critics say could further chill an already recessionary economy. Administration officials insist they'd prefer to let Congress set those limits, and that they will help spur millions of clean-energy jobs in the years to come.

Environmentalists are celebrating the so-called "endangerment finding" as the biggest statement yet that the federal government, after years of downplaying the dangers of climate change under the Bush administration, is now preparing to act boldly to combat it.

The is a "landmark moment in environmental history," Frank O'Donnell, president of the environmental group Clean Air Watch, said in a press release anticipating the decision.

"Where the Bush administration lagged, the Obama administration is now leading," said David Bookbinder, Sierra Club chief climate counsel. "There is no longer a question of if or even when the U.S. will act on global warming. We are doing so now."

Critics warn that the policy could cripple small businesses and kill economic growth.

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