When President Trump’s cabinet secretaries clashed with Elon Musk this month over the billionaire’s chain saw approach to shrinking government, Lee Zeldin, the administrator of the Environmental Protection Agency, made it clear where he stood.

Mr. Musk had just traded barbs with Sean Duffy, the transportation secretary, and Marco Rubio, the secretary of state. Things were tense. That’s when Mr. Zeldin chimed in to say he had no complaints about Mr. Musk, according to three people briefed on the March 6 meeting.

It was a telling moment for Mr. Zeldin, who has evolved from moderate blue state Republican to full MAGA warrior, taken to dismantling the agency he oversees with zeal.

Over the past nine weeks, Mr. Zeldin has withheld billions of dollars in climate funds approved by Congress, tried to fire hundreds of employees, recommended the elimination of thousands more E.P.A. scientists, and started trying to repeal dozens of environmental regulations that limit toxic pollution. He has filled the leadership ranks at the agency with lobbyists and lawyers from industries that have fought environmental regulations.

He has embraced Mr. Musk, calling theirs “an incredible partnership” and requiring that any expenditure at the E.P.A. over $50,000 be approved by Mr. Musk’s advisory group, the Department of Government Efficiency. Two members of the DOGE team, Kathryn Loving and Cole Killian, occupy an office outside Mr. Zeldin’s wood-paneled executive suite on the third floor of the E.P.A. headquarters. A handwritten note was recently taped to the door that said “Welcome to DOGE.”

Mr. Zeldin, 45, has canceled programs focused on marginalized communities that are disproportionately burdened by pollution, including an area in Louisiana known as Cancer Alley. He says he wants clean air and water, yet is taking steps scientists say will endanger both, while also working to ensure that the E.P.A. can never again limit the greenhouse gases that are dangerously heating the planet.