This story was originally published by Mother Jones and is reproduced here as part of the Climate Desk collaboration.

President Obama’s signature climate change initiative had its day in court Tuesday, as lawyers for 27 states, nonprofit groups, and utility companies argued that it is unconstitutional.

The rule, known as the Clean Power Plan, would enforce a 32 percent reduction in greenhouse gas emissions from electric power plants by 2030 (compared with 2005 levels). As part of the implementation, the Environmental Protection Agency would require states with at least two coal-fired power plants to submit plans for emissions reductions. If a state chose not to submit an acceptable plan, the EPA would impose one on it. The plan was a critical piece of the Obama administration’s successful efforts to forge the landmark Paris climate agreement last year.