The Supreme Court on Tuesday blocked a key part of President Obama’s ambitious proposal to limit carbon emissions and reduce global warming while the plan is challenged.

The court granted a stay request from more than two dozen states, plus utilities and coal companies, that said the Environmental Protection Agency was overstepping its powers. The court’s decision does not address the merits of the challenge but indicates justices think the states have raised serious questions.

The administration’s initiative, which is still in the planning stages, required states to submit plans for shifting away from fossil-fuel power plants in favor of alternative forms of energy. It is aimed at reducing emissions of carbon dioxide at existing plants by about a third by 2030.

The stay means that questions about the legality of the program will remain after Obama leaves office. An appeals court is not scheduled to hear the case until June, and the Supreme Court’s order said the stay would remain in effect while the losing side petitions the Supreme Court. If the court were to accept the case, that would mean an ultimate decision in 2017.

The timing imperils the Clean Power Plan, because a new president could make significant changes.

An ultra-high-resolution NASA computer model has given scientists a stunning new look at how carbon dioxide in the atmosphere travels around the globe. (NASA's Goddard Space Flight Center)

White House press secretary Josh Earnest said the administration disagrees with the court’s decision. “We remain confident that we will prevail on the merits,” he said in a statement. “Even while the litigation proceeds, EPA has indicated it will work with states that choose to continue plan development and will prepare the tools those states will need. At the same time, the Administration will continue to take aggressive steps to make forward progress to reduce carbon emissions.”

As is its custom in stay requests, the court did not give a reason for its action. The court’s four liberal justices — Ruth Bader Ginsburg, Stephen G. Breyer, Sonia Sotomayor and Elena Kagan — objected to the decision, but they did not give an explanation.

The Obama administration had told the court that the stay request was unprecedented and that it was routine for federal programs to proceed while courts considered challenges.

West Virginia Attorney General Patrick Morrisey, who spearheaded the states’ stay request, said the decision showed the court was leery of the plan.

“We are thrilled that the Supreme Court realized the rule’s immediate impact and froze its implementation, protecting workers and saving countless dollars as our fight against its legality continues,” Morrisey said in a statement.

Environmentalists said they hoped the setback was only temporary.

“Today’s court decision is unfortunate but does not reflect a decision on the merits,” said Vickie Patton, general counsel for the Environmental Defense Fund. “The D.C. Circuit Court will carry out a careful and expeditious review of the merits over the next few months. The Clean Power Plan has a firm anchor in our nation’s clean-air laws and a strong scientific record.” Jeffrey Connor, chief executive of the National Rural Electric Cooperative Association, said the existing plan would have caused “immediate and irreparable harm” to small electricity providers who rely on coal.

“Had the stay not been granted, co-ops would have been forced to take costly and irreversible steps to comply with the rule, which is a huge overreach of EPA’s legal authority,” Connor said.

The Clean Power Plan is an essential part of Obama’s pledge to reduce the country’s contribution to global warming, because the electric-power sector of the economy emits 30 to 40 percent of all U.S. greenhouse gases.

Under the plan, states can draw up their own plans or choose the EPA’s plan. The EPA recently published its proposals and is finalizing that rule now. States were supposed to submit their plans by September but were allowed to ask for a two-year extension.

“It won’t prevent the EPA from doing the kind of planning they want to do, but most of the states are putting their pencils down,” said Jeffrey R. Holmstead, a senior EPA official under President George W. Bush and a critic of the Clean Power Plan. Some experts were surprised at the court’s action. It rarely intervenes to halt a regulation that had not already been reviewed by a lower court, said Nathan Richardson, a University of South Carolina law professor.

“The bigger signal here is that there’s a lot of skepticism from the Supreme Court,” said Richardson, a visiting fellow at Resources for the Future, a nonpartisan think-tank. “You’re getting an earlier view of how the justices feel.”

“We’re obviously very disappointed,” said Bruce Nilles, a Sierra Club lawyer who has been campaigning to close down coal plants for years. “Almost every state has been in the process of planning for an orderly transition of cutting carbon and replacing coal plants with clean energy.”Nilles, a former Justice Department lawyer, said: “It is unprecedented for the Supreme Court to stay a rule at this point in litigation. They do this in death-penalty cases.”But the states had argued that the court needed to act now. Without a stay, they told the court, the plan would “force massive, irreversible changes in terms of state policies and resources, power plant shutdowns, and investments in wind and solar power” all in the name of a program that might ultimately be found unconstitutional. Other experts said that many states would likely continue steps to cut greenhouse gas emissions without waiting for the legal fight to shake out. A number of states were already on track to meet the EPA standards because of commitments to phase out older coal-burning power plants and develop clean-energy projects.

But Nilles said that coal plants would still continue to close down because of low natural gas costs and declining costs of wind and solar power.

“Whether or not the court ultimately upholds this particular rule, the need to cut carbon emissions will remain, and states need to figure out the most cost-effective ways to do that,” said Bob Perciasepe, a former acting administrator for the EPA. “It’s in everyone’s interest that states keep at it, because whether it’s the Clean Power Plan or some other policy, they’ll need smart strategies to get the job done.”

Juliet Eilperin and Joby Warrick contributed to this report.