An environmental group that has sued the Trump administration nearly 100 times announced Tuesday that it has hired Gina McCarthy, who headed the Environmental Protection Agency under President Barack Obama, as its new president and chief executive.

In a phone interview Tuesday, McCarthy, who headed the EPA from July 2013 until January 2017, said she was joining the Natural Resources Defense Council to fight for stronger environmental protections and address the challenge of climate change.

“I really didn’t want to sit on the sidelines. So the question was where to go,” she said, adding that the current administration “is trying to undo not only fundamental protections that have been in place under multiple administrations, but it’s undermining the science and the work of scientists to protect the environment and the public.”

She added that she’s particularly concerned about policies aimed at limiting the kind of scientific studies that can be used in rulemaking and those that change the makeup of scientific advisory boards, as well as plans to relocate agencies outside Washington. “It’s not as much about the rollbacks, but it’s diminishing the capacity of federal agencies to do their job.”

McCarthy, who served as the EPA’s air policy chief for four years before becoming administrator, helped craft many of the landmark climate policies that the current administration is unraveling. She also worked on the 2015 Paris climate accord, which the United States formally began withdrawing from Monday.

Since President Trump has taken office, the NRDC has emerged as one of the most aggressive groups challenging his administration’s work to roll back federal climate rules and bolster policies aimed at promoting energy exploration and other forms of development. It has sued the federal government 96 times on issues such as endangered species, energy efficiency standards for lightbulbs and more. According to the group, it has won 54 of the 59 cases that have been resolved so far.

McCarthy, a 65-year-old Massachusetts native who was hailed by conservationists but denounced by many energy industry officials during her time in office, has criticized Trump and his deputies. After leaving the Obama administration, she joined the faculty of Harvard University’s T.H. Chan School of Public Health as a public health professor.

Alan Horn, who chairs the NRDC’s Board of Trustees, said in a statement: “Gina McCarthy is one of the most effective environmental champions of our time. She knows better than anyone what we can — and must — do to combat climate change and ensure all communities can thrive.”

Western Energy Alliance President Kathleen Sgamma, whose group represents roughly 300 oil and gas firms, said in an email: “It’s not surprising that a former EPA head is taking the job as a top environmental lobbyist. We wish her the best of luck.”

Asked whether her decision to join the NRDC reflects the close relationship and access environmental groups had to Obama officials, McCarthy said that the organization sued her multiple times even though they shared similar goals.

“They found ways to sue me all the time, which is probably a good thing,” she said, adding that her decision to join the staff was based on its 50-year advocacy record. “It wasn’t about prior relationships. It was about prior history.”

Several industry officials who opposed many of the EPA’s policies during McCarthy’s time as administrator praised her as a savvy strategist. Stephen Brown, an energy consultant with RBJ Strategies, said in an email that fossil fuel companies should be prepared to do battle.

“Getting her is the smartest thing NRDC has done in years, and the takeaway for those still on the sidelines of the climate debate should be, ‘Game on,’” he said.