In the past three months, regulators appointed by President Donald Trump have disrupted ambitious plans to combat climate change in electric grids serving 85 million people in the U.S., from Chicago to New York to Washington. It was easy: an agency just rewrote some obscure pricing rules.

With that, the Federal Energy Regulatory Commission cranked up to a new boil the simmering debate over whether the U.S. should get its electricity from fossil fuels or sources that don’t spew carbon. The commission’s Republican majority decided the playing field should be leveled so no one generator gets special treatment.

“They’re taking these markets in a totally different direction than states want to go,” said Ari Peskoe, director of the Electricity Law Initiative at Harvard Law School. “It could backfire quickly.” The rulings set up a battle over whether left-leaning states including New York, New Jersey and Illinois can effectively promote clean power. Wind and solar have long depended on state quotas and subsidies for growth, and those incentives could now be hobbled.