California has vowed to challenge this move in court, and some automakers have expressed unease at a legal fight that could drag on for years and potentially fracture the nation’s vehicle market. One question is whether Mr. Wheeler will try to negotiate a compromise with California and other states in order to avoid risky litigation.

Image Andrew Wheeler will become the E.P.A.’s acting administrator on Monday. Credit... Environmental Protection Agency, via Reuters

“This will be an early test,” said Jody Freeman, a law professor at Harvard who was the counselor for energy and climate change in the Obama White House. “Does he follow Pruitt and take a big legal risk by aggressively going after California? Or does he try to pull back in search of a deregulatory result that everyone can live with?”

3. Scaling Back the Clean Water Rule

Last year, Mr. Pruitt signed a proposal to pare back an Obama-era regulation known as the Waters of the United States rule, which sought to clarify which streams and wetlands get automatic protection under the Clean Water Act. Farmers and developers had criticized the Obama-era policy as overly intrusive, and Mr. Pruitt sought to suspend the rule while writing a new, much narrower regulation that would extend protections to fewer waterways.

But that proposal faces an uncertain fate in the courts: In drafting a replacement, Mr. Pruitt’s E.P.A. planned to follow guidelines laid out by Justice Antonin Scalia in 2006 — in an opinion that did not receive majority support on the Supreme Court. Mr. Wheeler will be tasked with trying to write a regulation that is legally defensible.

4. Changing the E.P.A.’s Use of Science

In April, Mr. Pruitt unveiled a proposal to change the way the E.P.A. relies on scientific research, by limiting the use of studies in which the underlying data is not publicly available. Scientific researchers criticized the move, noting that the proposal could exclude some of the most important studies available on the harms from air pollution or pesticides, because those studies frequently redacted confidential health information about their participants.

Mr. Wheeler is expected to move forward on this policy, but he may be forced to make changes. Several business groups, including pesticide makers and the National Association of Home Builders, recently expressed worry that Mr. Pruitt’s proposal was overly broad.