A 25-year veteran of environmental policy and politics, McCarthy has worked for Republicans and Democrats, including Obama’s presidential rival, Mitt Romney, who tapped her to help draft state plans for curbing the pollution linked to global warming. Along with McCarthy, Obama nominated MIT nuclear physicist Ernie Moniz to lead the Energy Department and Wal-Mart’s Sylvia Mathews Burwell to head the budget office.

McCarthy, 58, a Boston native, has led the EPA’s air pollution division since 2009, ushering in a host of new rules targeting air pollution from power plants, automobiles, and oil and gas production.

In nominating McCarthy as the nation’s top environmental official, Obama is promoting a climate change champion at a time when he has renewed his commitment to address global warming and the agency is contemplating a host of new rules that could help achieve that. But McCarthy will have to balance the administration’s ambitions with a dwindling budget: Congress has cut EPA’s budget by 18 percent over the last two years, and the automatic budget cuts that went into effect Friday will hinder the agency’s energy efficiency programs and climate research.

Moniz, as head of MIT’s Energy Initiative, has worked on developing ways to produce power while curbing greenhouse gas emissions.

“They’re going to be making sure we’re investing in American energy, that we’re doing everything we can to combat the threat of climate change, that we’re going to be creating jobs and economic opportunity,” Obama said.

McCarthy also brings a distinctive pronunciation of carbon dioxide, the chief pollutant blamed for climate change. McCarthy, in her thick accent, pronounces carbon as “cahbon.”

“You wouldn’t know by talking to her, but Gina’s from Boston,” Obama said. He then praised her for putting in place over the last four years what he said were “practical, cost-effective ways to keep our air clean and our economy growing.”

Already, McCarthy has orchestrated many of the agency’s most controversial new rules, such as placing the first-ever limits on greenhouse gases on newly built power plants and a long-overdue standard to control toxic mercury pollution from burning coal for electricity. On her plate, should she be confirmed by the Senate, will be even more rules — from lowering sulfur emissions from gasoline to controlling global warming pollution from the older coal-fired power plants.

Like those regulations, her nomination is all but guaranteed to spark criticism from Republicans, who charge that the agency is killing jobs and undermining the coal industry. Environmentalists, meanwhile, will be looking to ensure that McCarthy issues the toughest rules possible, particularly when it comes to controlling emissions from the existing fleet of power plants.

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 on 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 as 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.

Burwell is Washington veteran, having served in several posts during the Clinton administration, including deputy OMB director. She currently heads the Wal-Mart Foundation, the philanthropic arm of the retail giant, and previously served as president of the Gates Foundation’s Global Development Program.