Mr. Pruitt, a Republican, has been a key architect of the legal battle against Mr. Obama’s climate change policies, actions that fit with the president-elect’s comments during the campaign. Mr. Trump has criticized the established science of human-caused global warming as a hoax, vowed to “cancel” the Paris accord committing nearly every nation to taking action to fight climate change, and attacked Mr. Obama’s signature global warming policy, the Clean Power Plan, as a “war on coal.”

Mr. Pruitt, 48, who has emerged as a hero to conservative activists, is also one of a number of Republican attorneys general who have formed an alliance with some of the nation’s top energy producers to push back against the Obama regulatory agenda, a 2014 investigation by The New York Times revealed.

At the heart of Mr. Obama’s efforts to tackle climate change are a collection of E.P.A. regulations aimed at forcing power plants to significantly reduce their emissions of planet-warming carbon dioxide pollution. It will not be possible for Mr. Trump to unilaterally cancel the rules, which were released under the 1970 Clean Air Act. But it would be possible for a legally experienced E.P.A. chief to substantially weaken, delay or slowly dismantle them.