The Environmental Protection Agency said Monday it will ask the public for input on how to replace the Clean Power Plan, the Obama administration's key regulation aimed at cutting greenhouse gas emissions from power plants.

The main effect may be to leave the Obama rule in limbo. The Clean Power Plan was put on hold by the Supreme Court pending litigation that was under way before Donald Trump took office on a promise to undo it.

In an "advanced notice of proposed rulemaking"—a first step in the long process of crafting regulation—the EPA said it is "soliciting information on the proper and respective roles of the state and federal governments" in setting emissions limits on greenhouse gases.

In October, the agency took the first step toward repealing the rule altogether, but that has raised the prospect of yet more legal challenges and prompted debate within the administration over how, exactly, to fulfill its obligation to regulate greenhouse gases.

The Supreme Court has ruled that the agency is required to regulate greenhouse gas emissions in some fashion because of the "endangerment finding," a 2009 ruling that called carbon dioxide a threat to public health and forms the basis of the Clean Power Plan and other greenhouse gas regulations.

EPA Administrator Scott Pruitt has said he wants to repeal the Obama plan, but it's clear the agency is also weighing replacement options—options that would weaken regulations. The Clean Power Plan allows states to design their own strategies for cutting emissions, but Monday's notice signals that the Trump EPA believes states have "considerable flexibility" in implementing emissions-cutting plans and, in some cases, can make them less stringent.

In any case, the latest notice suggests an attempt to "slow-walk" any new regulation.

"Though the law says EPA must move forward to curb the carbon pollution that is fueling climate change, the agency is stubbornly marching backwards," Earthjustice President Trip Van Noppen said in a statement. "Even as EPA actively works towards finalizing its misguided October proposal to repeal the Clean Power Plan, EPA today indicates it may not put anything at all in the Plan's place—or may delay for years and issue a do-nothing substitute that won't make meaningful cuts in the carbon pollution that's driving dangerous climate change."

The goal of the Clean Power Plan is to cut carbon dioxide emissions from power plants 32 percent below 2005 levels, a target that is central to the United States' commitment to reduce greenhouse gas emissions under the 2015 Paris climate agreement.

Twenty-eights states have challenged the regulation, which is now stalled in federal appeals court.

"They should be strengthening, not killing, this commonsense strategy to curb the power plant carbon pollution fueling dangerous climate change," David Doniger, director of the climate and clean air program at the Natural Resources Defense Council, said in a statement. "A weaker replacement of the Clean Power Plan is a non-starter. Americans—who depend on EPA to protect their health and climate—deserve real solutions, not scams."

In an emailed statement Monday, Pruitt noted that the agency is already reviewing what he called the "questionable legal basis" of the Obama administration's plan. "Today's move ensures adequate and early opportunity for public comment from all stakeholders about next steps the agency might take to limit greenhouse gases from stationary sources, in a way that properly stays within the law and the bounds of the authority provide to EPA by Congress."