Had Donald Trump spent an entire year scouring the country for someone to weaken clean air and clean water laws and repudiate America’s leadership role in the global battle against climate change, he could not have found a more suitable candidate than Scott Pruitt, the Oklahoma attorney general, whom he picked on Wednesday to run the Environmental Protection Agency.

This is an aggressively bad choice, a poke in the eye to a long history of bipartisan cooperation on environmental issues, to a nation that has come to depend on the agency for healthy air and drinkable water, and to 195 countries that agreed in Paris last year to reduce their emissions of climate-changing greenhouse gases in the belief that the United States would show the way. A meeting Monday between Mr. Trump and Al Gore had raised hope among some that the president-elect might reverse his campaign pledge to withdraw the United States from the Paris accord. The Pruitt appointment says otherwise.

Since becoming Oklahoma’s top legal officer in 2011, Mr. Pruitt has been a bitter opponent of the E.P.A., joining in one lawsuit after another to kill off federal environmental regulations. He has challenged standards for reducing soot and smog pollution that cross state lines. He has fought protections against mercury, arsenic and other toxic pollutants from power plants. He has sued to overturn an E.P.A. rule modestly enlarging the scope of the Clean Water Act to protect streams and wetlands vital to the nation’s water supply.

More recently — and of greater interest to the world community — he has joined with other states in a coordinated effort to overturn the E.P.A.’s Clean Power Plan, the centerpiece of President Obama’s regulatory efforts to reduce carbon pollution. If approved by a federal court, the plan could transform the electricity sector, close down hundreds of coal-fired power plants and encourage the growth of cleaner energy sources like wind and solar.