The U.S. Court of Appeals in Washington, DC will hear four hours of oral arguments on Tuesday about the Clean Power Plan, the Obama administration’s contentious policy to curb greenhouse gas emissions from power plants. The idea is to get power plant operators to reduce emissions by turning to cleaner-burning natural gas, and carbon emissions-free solar and wind energy, along with shuttering some of the dirtiest coal plants.

Twenty eight states and over a 100 industry groups, many of which representing the interests of the coal industry, are calling for the plan to be shot down. Eighteen states and dozens of environmental and clean power lobbying groups are meanwhile defending it.

While the plan’s ultimate fate will likely be decided later by the Supreme Court, the oral arguments on Tuesday will highlight the key arguments for and against the Obama administration and the Environmental Protection Agency’s interpretation of the law.

For more on Obama’s Clean Power Plan watch our video.

The plan’s biggest impact will likely be an acceleration of the closures of older U.S. coal plants, and a simultaneous increase in both new natural gas power plants and solar and wind farms. Not surprisingly, some of the plan’s staunchest opponents include coal companies that have been hit hard in recent years, like Peabody Energy, which filed for bankruptcy protection in April.

The plan is an extremely important part of Obama’s environmental legacy, which has helped the U.S. make commitments to reduce greenhouse gas emissions through what’s known as the Paris agreement, an accord that includes pledges from countries around the world. The U.S. and China ratified those agreements this weekend.

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At the same time, the court case has some unusually personal elements, notes the New York Times. Merrick Garland, the Obama Supreme Court nominee who Republicans have denied confirmation hearings, is the chief judge of the Appeals court, and thus has recused himself. Obama’s mentor at Harvard Law School, Laurence Tribe, is arguing against the plan for coal company Peabody Energy. Peabody Energy is reportedly paying Tribe $435,000 this year for his help challenging the policy.

Democratic presidential nominee Hillary Clinton has said she supports the Clean Power Plan and that she will continue largely on the same path as the Obama administration by pushing for more clean energy and putting in policies to fight climate change. At the same time, Republican Donald Trump has said he will eliminate the Clean Power Plan and repeal other energy regulations.