President Obama’s signature climate change initiative is about to face its biggest challenge yet.

Time is running out on Obama’s second term in the White House, and the president could leave office next January with his Clean Power Plan stuck in legal limbo. 

{mosads}The rule, which would require power plants to cut their carbon dioxide output, is the centerpiece of Obama’s efforts to use executive power to slow U.S. contributions to global warming.

It was the main pillar in the president’s pledge to cut greenhouse gas emissions under the Paris climate agreement, the first time nearly every country in the world committed to address the causes of climate change together.

The climate deal will be a significant part of Obama’s environmental legacy, and it is now hanging in the balance.

More than two dozen states and a collection of industry groups have filed lawsuits against the plan, arguing the Environmental Protection Agency’s (EPA) rule oversteps its authority. 

The Supreme Court voted 5-4 in February to put the plan on hold until the lawsuit works its way through the legal system. That could keep the rule on ice for well over a year.

Complicating the legal wrangling further is the high-stakes political fight to replace Supreme Court Justice Antonin Scalia, who died in February.

Scalia’s replacement could swing the court to the left or the right ideologically. With Senate Republicans promising to block anyone Obama nominates, that means the 2016 presidential election could effectively decide the court’s majority. In the meantime, the Supreme Court will have just eight justices.

If the case lands in the Supreme Court before a ninth justice is confirmed, there is a good chance of a 4-4 split. That would leave the lower court’s ruling in place. 

That raises the stakes for the D.C. Circuit Court of Appeals, which is expected to hear oral arguments this June and hand down a decision in September or October. 

On the surface, the three-judge panel appears likely to side with the Obama administration’s argument that the rule falls under the EPA’s authority under the Clean Air Act.  

Judge Judith Rogers, a Democratic appointee, has a track record of defending the EPA’s authority. Several lawyers following the case see Karen L. Henderson, a GOP appointee, as a vote against the Obama administration. 

That leaves Judge Sri Srinivasan, whom Obama considered to replace Scalia, as the pivotal vote.

Srinivasan is an Obama appointee but has a short paper trail on the bench after having served less than three years on the D.C. Circuit. 

In private practice, he defended ExxonMobil and mining company Rio Tinto in human rights cases. He also represented former Enron CEO Jeffrey Skilling in an appeal of his fraud conviction. 

“Sri Srinivasan — he’s thought of as a wild card on this,” said William Yeatman, a senior fellow at the Competitive Enterprise Institute, which is part of a lawsuit challenging the EPA rule.

“I think that people shouldn’t be too pessimistic or optimistic either way,” said Jody Freeman, director of Harvard Law School’s environmental law program and a former counselor to Obama. She filed a legal brief backing the plan on behalf of two former Republican EPA chiefs.

“Just because you’re appointed by a Democrat doesn’t mean we know which way you’re going to rule,” he added.

The case poses unprecedented questions for the judges.

The legal justification for the Clean Power Plan lies in three separate Supreme Court rulings since 2007 that have declared the EPA has the power to regulate greenhouse gas emissions under a section of the Clean Air Act. 

But the many groups challenging the plan argue Obama’s plan is unprecedented in its scope and goes far beyond the agency’s authority.

The most novel legal argument stems from a peculiar action Congress took more than a quarter century ago. The House and Senate passed two conflicting amendments to the provision of the Clean Air Act used as the legal underpinning of the Clean Power Plan. 

A House amendment passed in 1990 can be interpreted as barring the agency from imposing new regulations on existing power plants already regulated by other Clean Air Act provisions listing hazardous air pollutants.

But the Senate version does not impose the restriction. 

Supporters of the Clean Power Plan believe the EPA’s effort to reconcile the texts in the final rule will win the day. But they’re not sure if the judges will see it the same way. 

“It’s a question we haven’t seen before,” said Freeman. “Nobody really knows what the court might do with that.”

In the end, the court battle could be moot after the November elections. 

If Republicans take control of the White House, the next president would likely scrap the Clean Power Plan.

Republican candidates Ted Cruz and John Kasich have railed against the rule. GOP front-runner Donald Trump has said he wants to eliminate the EPA altogether. 

It’s a reminder of how fragile Obama’s legacy on climate change is. Congress’s failure to pass cap-and-trade legislation early in his administration means much of it is tied up in executive actions that can be reversed by his successor.

That possibility could undermine the president’s other major climate achievement, the Paris agreement. The Clean Power Plan would drive Obama’s commitment to cut U.S. emissions by up to 28 percent by 2025.

“The international community seems to be taking a wait-and-see approach,” said Freeman. “The administration is emphasizing we are on track to meet the target of being 17 percent below 2005 emissions levels by 2020.  And they are hoping and counting on the international community saying that is a good-faith process.

“The prospect of new leadership that will turn its back on international climate agreements is far more concerning.”