California asked the new Environmental Protection Agency chief to revisit the Bush administration's 2007 decision that denied the state's request to impose its own regulations.

"We think we should have our decision in hand by late May," Mary Nichols, California's top air quality regulator, said in an interview. Nichols, the state Air Resources Board chairman, sent a letter to designated EPA Administrator Lisa Jackson on Wednesday requesting the review.

If the EPA reverses the Bush administration ruling, more than 12 U.S. states could proceed with plans to impose stricter limits on greenhouse gas emissions from cars. California wants to reduce the emissions by 30 percent by 2016 -- the most ambitious federal or state effort to address global warming.

Environmentalists have said granting California a waiver that allows it to set its own emissions regulations would signal Obama's commitment to tackling climate change.

California wants to require carmakers to use paints that reflect more heat, tires that roll smoother and improved air conditioning to boost efficiency beyond the fuel mileage requirements already facing automakers.

EPA officials were not immediately available for comment.

'ALLIES IN OUR EFFORTS'

President George W. Bush's administration denied the request by California, which needs federal clearance to set clean air standards alongside the U.S. government.

Jackson has said she that she would revisit the so-called California waiver. She comes from New Jersey, which is one of more than a dozen states that will follow California's lead if the EPA grants the waiver.

"We've been allies in our efforts," Nichols said.

It would take the federal government until May to clear several procedural hurdles, and then California could proceed, she predicted.

"If the California waiver is granted, states that represent over half the population of the United States and an even larger part of the market for new cars will be committing themselves to require the auto manufacturers to produce and sell vehicles that are 30 percent cleaner," she added.

Bush often drew criticism from environmentalists, who said his administration favored industry and politics over environmental science. His EPA administrator came under fire after he denied California's request. California and other states sued over the decision.

Automakers have said the changes could add substantially to sticker prices. Nichols said it would add a little over $100 to the price of car, and the improvements would pay off within a year through improved efficiency.

In a letter to Obama, California Gov. Arnold Schwarzenegger also pressed for the EPA to reconsider.

"I ask that you direct the U.S. EPA to act promptly and favorably on California's reconsideration request so that we may continue the critical work of reducing our greenhouse gas emissions and their impact on global climate change," Schwarzenegger wrote.