Gina McCarthy, center, has been nominated by President Obama to head the Environmental Protection Agency. (Win McNamee/Getty Images)

In March 2009, public health advocates S. William Becker and Frank O’Donnell were walking the halls of the Environmental Protection Agency when they spotted Gina McCarthy poring over briefing books. She was preparing for her Senate confirmation hearing as head of the agency’s air and radiation office, and she was intent on defusing any controversy surrounding global warming.

“I know greenhouse gases are important, but I’m committed to strengthening public health protections,” she told them.

It was an early indication of how the politically savvy McCarthy, whom President Obama tapped Monday to head the EPA, was prepared to make her way in Washington. A career environmental administrator and veteran of Republican administrations in Massachusetts and Connecticut, she has devoted much of the past four years to shepherding through air regulations that have protected public health — but also have helped close power plants emitting greenhouse gases linked to climate change.

“She’s very data- and fact-driven, and that’s been helpful for us as well as the entire business community,” said Donna Harman, president and chief executive of the American Forest and Paper Association. “It doesn’t mean I always got what I was looking for, but we can have a dialogue.”

Many environmentalists, for their part, see McCarthy’s ability to broker deals with potential opponents as an asset. “What she’s tough about is the science-based standard,” said Kevin Knobloch, president of the Union of Concerned Scientists. “She’s very pragmatic about how you get there.”

Still, the nomination of McCarthy, 58, comes amid questions about the EPA’s responsiveness to lawmakers’ requests and its sensitivity to economic factors. Climate-change policy has become more polarized since Obama took office and will come under intense scrutiny now that he has identified it as a top priority for his second term.

For four years, the EPA and the woman who led it until last month — Lisa P. Jackson — have been a lightning rod for criticism, with many manufacturing and utility executives accusing it of imposing job-killing regulations. The tensions have run so high that McCarthy, speaking at a luncheon in Kentucky in 2011, felt compelled to say, “The EPA is not the enemy.”

Now she will have to prove it to the agency’s critics. Sen. Barbara Boxer (D-Calif.), who chairs the Senate Environment and Public Works Committee, said McCarthy’s “experience, intelligence, energy, and unquestioned expertise” make her “the right person for the job,” but some Republican senators have expressed reservations.

“The EPA is in desperate need of a leader who will stop ignoring congressional information requests, hiding emails and more from the public, and relying on flawed science,” Sen. David Vitter (La.), Boxer’s GOP counterpart on the panel, said in a statement.“I look forward to hearing answers from her on a number of key issues.”

Coal companies have also privately expressed reservations about McCarthy, though none responded to requests for comment.

American Chemistry Council President Cal Dooley, who said he and his group’s members “have a lot of confidence in McCarthy’s leadership ability,” noted that her division oversaw the implementation of several of the EPA’s most controversial regulations over the past four years, including greenhouse gas limits for new power plants and curbs on mercury emissions from existing utilities.“She has been identified as the owner of those regulations,” he said.

McCarthy, a skilled pool player who grew up outside Boston and headed Connecticut’s Department of Energy & Environmental Protection before moving to the EPA, has won over many skeptics with her blunt, humorous style. She rarely reads speeches, and her tendency to crack jokes and deliver harsh comments in a heavy Boston accent can alleviate tension.

Bob Durand, who was Massachusetts’s secretary of environmental affairs under Gov. Paul Cellucci (R), said McCarthy picked her fights wisely while serving under him. She went after the state’s five biggest polluters in a campaign called “the Filthy Five,” helped assemble a coalition of sporting and environmental groups to pressure dentists and others to reduce mercury contamination under the banner “Mercury Is Rising,” and boosted the state’s solid-waste recycling rate by hashing out an agreement between environmental and business groups.

“I remember one of the pesticide guys telling me, ‘I can’t stand going in a room when Gina’s in there, because I know when I come out I’ll be mad, but I can’t get mad because I love Gina,’ ” said Durand, who now heads the consulting firm Durand & Anastas Environmental Strategies.