How the United States confronts climate change will be decided whenever the Democratic Party regains control of the White House and Congress. But it may ultimately be Chief Justice John Roberts who decides whether they succeed.

Every major Democratic policy would need to survive scrutiny by the justices. So how would the Green New Deal fare if it reaches the high court? This isn’t an easy question to answer, partly because lawmakers have yet to draft a proposal. Early last month, Representative Alexandria Ocasio-Cortez introduced a resolution recognizing “the duty of the Federal Government to create a Green New Deal”; 90 House Democrats have cosponsored it, and multiple Democratic presidential candidates have expressed support for it. But the resolution lacks the specificity of a bill. In fewer than 2,000 words, it outlines the Green New Deal’s goals, which include not only net-zero greenhouse gas emissions and renewable-energy investment but a job guarantee, universal healthcare, and more. 

While the resolution does not describe in detail how the government could achieve these goals, left-wing policy wonks have been working to fill in the gaps. Data for Progress, a progressive think tank with close ties to Ocasio-Cortez, released its own proposal for a Green New Deal last September. It provides a road map for what a legislative package could look like if Democrats win unified control in Washington—and thus serves as a useful guide for what the courts would have to wrestle with, once conservatives mount the inevitable legal challenges.

One of the Green New Deal’s key components is a familiar one: implementing the Clean Power Plan. The Obama administration unveiled the plan in 2014 to stave off a two-degree Celsius rise in global temperatures over the next century, which scientists warn would have a catastrophic impact on civilization. The plan called for drastically curbing greenhouse-gas emissions from power plants, namely those powered by coal, and was meant to persuade other countries to put forth their own plans ahead of the Paris climate summit later that year.

The plan was soon challenged in court, and in 2016 the Supreme Court blocked it from going into effect until the legal questions were resolved. Justice Antonin Scalia’s death a few days later effectively tied the plan’s long-term fate to the outcome of the presidential election later that year. After Donald Trump took office, his first EPA administrator, Scott Pruitt, began the process of repealing the Clean Power Plan. The agency’s proposed replacement, unveiled last August, would shift much of the discretion to regulate coal-plant emissions to the states. (The legal challenges to the plan are on hold until the replacement process is complete.) Data for Progress’s proposal would revive the plan, as well the legal challenges to it.

The think tank’s Green New Deal draws from other policy ideas developed under the Obama administration. Their paper recommends that the government “fully enforce the strengthened National Ambient Air Quality Standards Act of 2015”; that it “strengthen and enforce Obama-era rules on methane leakage”; that it “reinstate and implement the Obama-era ‘Clean Water Rule’ to limit pollution in a variety of streams, tributaries, and wetlands.” Conservative critics have warned that the Green New Deal would turn America into Cuba or Venezuela, but the end goal looks more like what the EPA was already doing before Trump took over.

The Roberts Court isn’t completely hostile to efforts to combat climate change, as environmentalists have won some key victories there over the past decade. During the George W. Bush administration, top EPA officials claimed they didn’t have the authority to regulate greenhouse gases as pollutants under the Clean Air Act. A coalition of states and environmental groups challenged that finding in federal courts. In the landmark 2007 case Massachusetts v. EPA, the Supreme Court ruled that greenhouse gases fell under the act’s broad definition of “air pollutants,” and that lawsuits against the EPA could go forward if the agency refused to regulate them. The justices later signed off on the Obama EPA’s first effort to regulate greenhouse gas emissions under the Clean Air Act.

Justice Anthony Kennedy, the court’s swing vote from 2005 to 2018, sided with the majority in both cases. But the rest of the court’s conservatives generally resisted those rulings. In Massachusetts v. EPA, Roberts questioned whether the states had standing to bring the case at all, arguing that Massachusetts’ claimed injury—the loss of coastal land from rising sea levels—couldn’t be directly tied to the EPA’s actions. And in the 2014 case, justices Samuel Alito and Clarence Thomas challenged the court’s earlier determination that the Clean Air Act authorized the EPA to regulate greenhouse gases at all.

Brett Kavanaugh’s confirmation to replace Kennedy last year raises the peril. Before joining the high court, Kavanaugh spent twelve years on the D.C. Circuit Court of Appeals, where he weighed in on multiple high-profile cases involving the EPA. In a survey of those decisions last year, The Atlantic’s Robinson Meyer found that the newest justice occasionally sided with the agency on major environmental cases. But Kavanaugh also often resisted the EPA’s efforts to break new regulatory ground when it came to curbing carbon emissions to fight climate change.

In 2014, for example, Kavanaugh served on a three-judge panel that heard White Stallion Energy Center v. EPA. The case centered on a slate of EPA regulations targeting mercury emissions at power plants. In a 2-1 decision, the panel sided with the agency after energy companies sued to block the rules. Kavanaugh, however, wrote a partial dissent where he argued that the EPA should have taken the costs of the companies’ compliance into the account when making its regulatory decisions. When the case—later retitled Michigan v. EPA—reached the Supreme Court, Scalia led the court’s conservative bloc to adopt Kavanaugh’s position.

Kavanaugh also appears set to take part in a mini-revolution on the court that could have far-reaching effects. Most of the EPA’s regulatory power comes from its interpretation of broad statutes like the Clean Air Act and the Clean Water Act. When federal courts interpret how agencies implement acts of Congress, they typically use what’s known as Chevron deference, so named for a 1980s case involving the oil giant. Generally speaking, the Chevron doctrine instructs judges to defer to a federal agency’s interpretation of federal law when deciding whether the agency is exceeding its authority under that law.

Over the past 30 years, Chevron deference evolved into a pillar of federal administrative law. It’s also recently become a target of ire among conservative legal scholars, who partially blame it for expanding the federal government’s regulatory powers. Justice Neil Gorsuch, a frequent critic of the doctrine, tilted the court’s balance on the subject when he replaced Scalia, who generally accepted it. Kavanaugh does not go quite as far as Gorsuch in his critiques, but his rulings and academic writings strongly suggest he’d vote to narrow the doctrine if given the opportunity. That could constrain Green New Deal–related regulations if the justices read the legislation more narrowly than the EPA staff does.

The Supreme Court has a spotty track record on upholding major legislation. The Court gutted Reconstruction-era laws to suppress Southern violence in the late nineteenth century, struck down key portions of the New Deal in the 1930s, and barely thwarted two major challenges to the Affordable Care Act during the Obama administration. With a conservative majority on the Court now all but guaranteed for the next 20 years, Democrats increasingly see the Roberts Court as a foe to be conquered rather than a Court to be persuaded.

Roberts’s vote to save the ACA’s individual mandate in 2012 is a touchstone for liberals who hope he may stave off the conservative bloc’s most sweeping decisions in the decades to come. The post-Kennedy court is still in its infancy, so it’s not yet clear whether Roberts is trending in that direction. But he may yet be called upon to do it again—this time with global stakes at hand.