The decision immediately provoked a debate over its scientific basis and whether the automobile industry applied political pressure to help it avoid the proposed regulations. Officials from the states and numerous environmental groups vowed to sue to overturn the edict.

Johnson defended the EPA's decision.

"The Bush administration is moving forward with a clear national solution, not a confusing patchwork of state rules," he said. "I believe this is a better approach than if individual states were to
act alone."

Led by California, the 17 states involved had waited two years for the Bush administration to issue a ruling on an application to set stricter air quality standards than those adopted by the federal government. The decision, technically known as a Clean Air Act waiver, was the first time California was refused permission to impose its own pollution rules; the federal government has previously granted the state more than 50 waivers.

The emissions standards California proposed in 2004 would have required automakers to cut greenhouse gas emissions by 30 percent in new cars and light trucks by 2016, with the cutbacks to begin in 2009 models. These reductions would have translated into roughly 43 miles per gallon for cars and some light trucks and about 27 miles per gallon for heavier trucks and sport utility vehicles.

The new federal law will require automakers to meet a 35-mile-per-gallon fleet wide standard for cars and trucks sold in the United States by 2020. It does not expressly address carbon dioxide emissions with the hope that such emissions would be reduced as cars become more fuel-efficient.

California's proposed rules sought to address the impact of carbon dioxide and other pollutants from cars and trucks that scientists say contribute to the warming of the planet. If the waiver had been granted for the 17 states, it would have covered at least half of all vehicles sold in the United States.