The issue focused on concerns over safety and pollution from coal burning power plants. As federal regulations grew stricter, states pushed back by suing to halt them. They argued that federal laws trumped state law and regulated everything from whether carbon dioxide emissions could be emitted in the first place to how long those emissions could remain in the atmosphere.

Since Obama took office in 2009, EPA rules have resulted in nearly half of all U.S. electricity generating capacity being powered by renewable energy sources such as wind and solar power.