“Some claim Mr. Pruitt opposes clean air and water. This could not be further from the truth,” wrote the groups, which include the political action committee Club for Growth; the American Energy Alliance, which has advised Mr. Trump on energy policy; and Americans for Tax Reform, the group founded by the anti-tax lobbyist Grover Norquist. Mr. Pruitt, the endorsement said, “understands that many of the nation’s challenges regarding clean air and water are best met at the state and local level.”

Mr. Pruitt, if confirmed, will take over the agency in an odd position: He has spent the last seven years suing it to block regulations that he would be expected to put into effect and enforce. Some legal scholars say he should recuse himself from major pending environmental matters, while groups like the Environmental Defense Fund are urging Congress to reject his confirmation.

“The president’s choices deserve a lot of deference from Congress and even environmental groups,” said Fred Krupp, president of the Environmental Defense Fund. “But at some point when the nominee has spent his entire career attempting to dismantle environmental protections, it becomes unacceptable. That’s why Mr. Pruitt is the first E.P.A. nominee from either party that the Environmental Defense Fund has opposed in our 50-year history.”

Some experts say that while returning more authority to states can be desirable in some cases, environmental protection is probably not one of them. Smog and toxic chemicals that foul the air and waterways of one state may originate from one or several others, necessitating federal oversight of pollution.

“Pollution doesn’t respect state boundaries,” said Patrick A. Parenteau, a professor of environmental law at Vermont Law School. “States have limited ability to regulate pollution from outside the state, and almost every state is downstream or downwind from other pollution.”

Case in point: the “Green Country” chicken battle that Mr. Pruitt inherited in eastern Oklahoma. The phosphorus and nitrates in chicken manure were causing algae blooms in the ponds, streams and lakes of the 1.1-million acre Illinois River watershed, which reaches from Arkansas into Oklahoma.