If the E.P.A. does open the door to a new, weaker set of rules that utilities and others favor, it will most likely touch off a legal battle with environmental groups and pose a bureaucratic challenge to an agency where critical senior positions remain vacant. It could also force the agency’s administrator, Scott Pruitt, who has rejected the scientific consensus that human emissions cause climate change, to implicitly acknowledge that greenhouse gases harm human health and that the E.P.A. has an obligation to regulate them.

“There’s an internal debate over what the overall approach toward greenhouse gases should be, and it’s hard to formulate policy if you haven’t come to terms with the outcome of the debate,” said David M. Konisky, a professor of public and environmental affairs at Indiana University.

The parallels between the Clean Power Plan and the Affordable Care Act go only so far. The health care law, which was passed by Congress, offered a tangible benefit to many Americans and was firmly in place when Mr. Trump entered office. The Clean Power Plan, a regulation, not legislation, has not taken effect and is tied up in a federal appeals court.

But environmental activists and conservative opponents alike say both cases show that demanding a policy be repealed is easier than making it happen. Finding a replacement is even harder.