Despite the partisanship in Washington, McCarthy has said the environment is a non-partisan issue, saying that the choice "doesn't have to be, 'Can I have a job or can I breathe clean air.'"
But she hasn't backed down when politicians have falsely portrayed her agency's work, such as suggesting EPA was poised to regulate cow flatulence to combat climate change and was looking to go after farmers for spilling milk.
"When I listen to their concerns, I am struck by the fact that what they think we are often doing bears little or no relationship to what we are actually doing," she said in testimony before Congress in April 2011.
Obama called her on Monday "a straight-shooter" who "welcomes different points of views."
Last year, the American Petroleum Institute praised an EPA rule for which she was responsible because it gave drillers two additional years to curb pollution from recently drilled oil and gas wells.
At the state level, McCarthy pressed for federal action to reduce greenhouse gases and was a key player in setting up the nation's first mandatory cap-and-trade system to reduce global warming pollution from power plants in 10 states. As head of Connecticut's environmental department, she is credited with convincing Republican Gov. Jodi Rell not to abolish a 10-state regional pact, even as other Republicans, including Romney, pulled out.
McCarthy was also Connecticut's point person on the environment when the state joined a lawsuit aimed at forcing the EPA to regulate global warming emissions from automobiles. When the Supreme Court ruled in April 2007 in the state's favor, McCarthy said "there's no downside." Many of the regulations she has helped shape at agency stemmed from that case.
But the state of Connecticut also sued the Bush administration for a limit on ground-level ozone, the primary ingredient in smog, which McCarthy believed was too weak. That standard is still in place, thanks to a decision by Obama to stall the fast-tracking of a stricter smog limit that had been drafted by McCarthy's division at EPA.
Environmentalists praised the nomination Monday, stressing her pragmatic approach to solving environmental problems and her ability to work with both parties.
Former Obama climate adviser and Clinton EPA administrator Carol Browner said in an interview that McCarthy has "a good understanding what the president needs to do, wants to do on climate change, which is to find the sweet spot for everyone, from the environmentalists to the states to companies."
But conservatives immediately stressed her role in what they view would be destructive policies from EPA.
"McCarthy will continue the regulatory attack on oil, coal and natural gas with the result that Americans will experience increasing energy costs and high unemployment rates," said Thomas Pyle, president of the Institute for Energy Research, a conservative think tank that receives some support from the fossil fuel industry.
Moniz, 68, was a former Energy Department undersecretary under Clinton. He's advised Obama on numerous energy topics, including how to handle the country's nuclear waste and the natural gas produced by the controversial technique of hydraulic fracturing.
Environmental groups are wary of Moniz, because of his support of natural gas and nuclear power. His MIT Energy Initiative has received funding from oil companies such as BP, Shell and Chevron.
Associated Press writers Matthew Daly and Julie Pace contributed to this report.