"There are a number of people who are creating a mystery novel when there's really no mystery to it," Johnson said in an interview.

He added that while he met with White House officials on the California issue last year, he "was not directed by anyone to make any decision."

Johnson said he made "a reasonable and fair evaluation of the information" and that critics suspicious of the process "just disagree" with the outcome.

Johnson's remarks came on the day that he issued a formal, 48-page justification to his controversial Dec. 17 decision to deny California and a dozen other states, including Pennsylvania and New Jersey, a waiver to create their own tailpipe emission rules.

California had sought the waiver under the Clean Air Act, citing "extraordinary and compelling conditions," including extreme fires, erosion and smog. But Johnson said the state's climate conditions weren't unique enough.

"While I find that the conditions related to global climate change in California are substantial," Johnson wrote in his formal decision, "they are not sufficiently different from conditions in the nation as a whole to justify separate state standards."

In the interview, Johnson said that the fact that a dozen other states wanted to follow California's proposed standards "really adds to my point - that in fact, California isn't unique."

"Simply put, the entire nation is facing the challenge of global climate change," he said.

In recent hearings, Senate Democrats have said that Johnson has used a thin legal argument to justify a position backed by automakers, who oppose tougher standards.

And, citing internal EPA documents, Democrats have said that Johnson ignored the advice of senior EPA scientists, who concluded that the California waiver was justified.

"It is shocking that even though the whole world now knows that the professional staff at EPA strongly urged Mr. Johnson to grant California's waiver, he completely walked away from his advisors, the science, and precedent, and did the bidding of the special interests instead," said Sen. Barbara Boxer, D-Calif.

Boxer, chair of the Senate Environment & Public Works Committee, has demanded that Johnson release related emails between senior EPA officials and the White House. Johnson has said Bush Administration officials are considering it.

Frank O'Donnell of Clean Air Watch said that Johnson's justification paper "reads like something written up in the boardroom of General Motors or a law firm working for car companies."

O'Donnell said that Johnson's focus on whether California is "unique" is legally flawed. He called it "a phony argument designed to protect the auto industry."

California and the other states, including Pennsylvania and New Jersey, last month sued EPA over its ruling, asking a federal court to force the agency to approve the waiver.

In the interview, Johnson said that his decision was not a popular one, but that it was correct and legal, based on the provisions and interpretations of the Clean Air Act.

Asked if California's more stringent approach to tailpipe emissions is simply a good idea - as a matter of policy, not law - Johnson demurred.

Changing the national tailpipe standard, he said, would take an act of Congress.

"I have to follow what the law says," he said. "It's not a popularity contest."