The head of the EPA stood firm Thursday against a chorus of congressional criticism over his refusal to allow California and more than a dozen other states to impose greenhouse gas reductions on cars and trucks.

"I am bound by the criteria in the Clean Air Act, not people's opinions," EPA Administrator Stephen L. Johnson testified to the Senate's environmental panel. It was his first congressional appearance since issuing the controversial waiver denial last month.

"The Clean Air Act does not require me to rubber-stamp waiver decisions," Johnson said. "It was my conclusion that California didn't meet the criteria, or at least all of the criteria."

Sen. Barbara Boxer, D-Calif., the committee chair, led committee Democrats in assailing Johnson's conclusion.

"You're going against your own agency's mission and you're fulfilling the mission of some special interests," she chided him.

California needs a federal waiver under the Clean Air Act to implement its first-in-the-nation tailpipe law, which would force automakers to cut greenhouse gas emissions by 30 percent in new cars and light trucks by 2016.

If California got the waiver other states could then impose the same rules. Twelve other states have already adopted them — Connecticut, Maine, Maryland, Massachusetts, New Jersey, New Mexico, New York, Oregon, Pennsylvania, Rhode Island, Vermont, and Washington — with others preparing to do so.

California and other states sued EPA earlier this month over Johnson's decision.

The EPA chief disputed Democratic suggestions that his decision was made under political pressure from the White House.

"I was not directed by anyone to make the decision, this was my decision," Johnson insisted.

He reiterated his position that it's better to have a single national standard for greenhouse gas emissions than different standards in different states, a position also supported by the auto industry.

Congress' newly passed fuel efficiency law — signed by President Bush last month on the same day Johnson announced he was denying the California waiver — provides such a national standard, he said.

Environmentalists contend that California's law is much stronger and takes effect much faster than the new federal rules.

Only one Republican attended Thursday's hearing, Sen. James Inhofe of Oklahoma, and he was the only senator to take Johnson's side. Inhofe, the committee's top Republican and a leading congressional skeptic of global warming, dismissed the proceedings as "more theater" and displayed a poster labeling California's law a "job killer" that he claimed would cost tens of thousands of auto industry jobs nationwide.

"I believe you're very courageous to be here today," Inhofe told Johnson.

The committee also heard from three governors who support California's position, Gov. Martin O'Malley of Maryland, Gov. Ed Rendell of Pennsylvania and Gov. Jim Douglas of Vermont. Almost alone in taking the opposite position was the attorney general of Michigan, Mike Cox, who argued that a national standard was needed and his state's auto industry was being singled out.

Cox and Rendell, sitting next to each other, got into a brief argument when Cox claimed that coal and other industries in Rendell's state were producing more greenhouse gas emissions without being forced to cut back. Rendell contended his state was cracking down on coal-fired power plants.