Washington - The Obama administration plans to announce on Tuesday that it will set national restrictions on greenhouse gas emissions from automobiles, in what environmentalists are hailing as a major step to curb global warming and spur development of more fuel-efficient cars.

    The national policy will mimic, with slight modifications, a California policy that state officials fought the Bush administration for years to implement, two sources with knowledge of the agreement said.

    California officials have signed on to the policy, one of the sources said. So have major U.S. automakers.

    Combining regulatory powers across the administration, the policy will toughen existing federal mileage standards with a harmonized standard that automakers and environmentalists have long sought -- and which administration officials have said for months they were working to set.

    "This is the biggest single step to curbing global warming," said Dan Becker, director of the Safe Climate Campaign, an environmental group. "It's a major step forward in cutting auto emissions, and California blazed the trail."

    The California rules don't strictly limit mileage. But, by setting caps on carbon emissions, they would effectively require vehicles to achieve as much as 42 miles per gallon by 2020, according to some estimates.

    One of the sources with knowledge of the national policy said it would emulate the California emissions-reduction targets, but modify how quickly those targets would need to be met.

    The national policy announcement will not affect California's request to set its own emissions standards, which is currently pending before the Environmental Protection Agency, the source said.

    California first attempted to regulate vehicle emissions in 2002. More than a dozen states also sought to adopt its standards. But the Bush administration denied their requests, a decision that President Obama ordered the EPA to revisit shortly after taking office.

    Obama's EPA also issued a draft ruling last month declaring that greenhouse gases pose a danger to public health and are subject to regulation under the Clean Air Act. The proposal singles out cars and trucks, which make up about one-quarter of the nation's greenhouse gas emissions, for regulation.

    Administration officials long have said they were drafting a single rule that would unite the greenhouse gas regulations and tougher fuel-economy standards, which Congress approved in 2007. The officials have cast such standards as a key to encouraging the deployment and sale of the fuel-sipping cars that Obama calls the key to the Detroit automakers' recovery -- and key to the international fight against global warming.

    In his announcement on federal support for the automakers in March, Obama declared: "I am absolutely committed to working with Congress and the auto companies to meet one goal: The United States of America will lead the world in building the next generation of clean cars."

    Auto companies and their congressional allies urged Obama to set a national emissions standard instead of allowing a "patchwork" of state laws. California officials agreed but said they wouldn't settle for any national standards that were weaker than their state's standards.

    "Our hope is that there would be a federal [standard] that we could sign onto and support," Mary Nichols, chairwoman of the California Air Resources Board, said in March. "That would be the best for the world."

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