Gray, the top EPA science adviser, said Johnson is a voracious reader and inquisitor. "We get down in the weeds," said Gray. "We part individual blades of grass."

But several other people who have worked closely with him say that too often politics trumped science. Former aides say the veteran scientist Johnson became subsumed by the rookie politician Johnson.

Four former Republican appointees said Johnson became so enamored of the perks of the job - trips to Camp David, flights on Air Force One, fireworks on the White House portico - that it affected his judgment, making him less likely to confront the White House.

"Here's a guy who labored in the bureaucracy for 25 years, then becomes assistant administrator, deputy, and then administrator, and all of a sudden he's being invited to spend the weekend at Camp David," said one senior Republican appointee. "I think it really did turn his head."

Johnson dismissed such talk. The perks, he said, come with a tough job.

"It's all part of the wow factor," he said. "While those are fun and exciting events - pinch-yourself type of events - the reality is that you've been asked to make very difficult decisions and are part of a team, a cabinet member."

Clearly, Johnson has been eager to execute the Bush agenda. John D. Graham, who was the regulatory guru of the White House Office of Management and Budget from 2001 to 2006, said one of Johnson's biggest accomplishments as assistant administrator was the repeal of a Clinton-era ban on human pesticide testing.

"Johnson faced a dilemma," said Graham, now dean of the Indiana University School of Public and Environmental Affairs.

The human testing studies were not perfect; some did not meet modern ethical standards, Graham said. But advocates were pushing for a ban regardless of their scientific value.

Johnson fixed that. He played a key role in reversing the ban, Graham said, fulfilling a goal shared by Bush and the pesticide industry.

"The White House loved Steve because he was the ultimate staffer," said another Republican colleague who worked at EPA. "He knew how to get things done."

When Whitman abruptly resigned in 2003, Johnson filled a void in the No. 2 slot - as acting deputy administrator - then remained when former Utah governor Michael Leavitt was named administrator. In 2005, when Leavitt got the job he really wanted - secretary of health and human services - Johnson became acting administrator. He lobbied for the top job, competing with three others.

Hazen said she warned her friend that the second Bush term would be fraught with complicated, nuanced decisions, and was destined to be controversial.

"My question to him was: Do you really want this? You've had a phenomenal career. You are well-respected. Do you really want to be the one making these hard decisions, charting new territory?"

Ethics 101

Ironically, Johnson's success at restoring human testing while assistant administrator nearly derailed his Senate confirmation for the top job.

One of the new human tests was the Children's Health Environmental Exposure Research Study (CHEERS). Funded with $2 million from the chemical industry, CHEERS proposed to record the effects of household pesticides on low-income children in Florida. EPA gave participating families $970, a video camera to record exposure, and a CHEERS T-shirt, calendar and baby bib. EPA scientists would collect urine samples and the children would wear a watch-size sensor one week each month.

Several Democrats were aghast. Boxer and other Democrats put a public hold on his nomination.

"Ethics 101: Testing pesticides on small children and infants is wrong," Boxer said. "This is sick. It's a sick, sick thing."

EPA officials said that the senators were overreacting, that CHEERS merely paid families already using pesticides to monitor their children. Johnson, who began his EPA career in the pesticide office, said that although EPA had no improper ethical intent, it could no longer overcome such an appearance.