A coalition of state attorneys general led by California sued the EPA and Administrator Scott Pruitt May 31, saying the delayed enforcement of emission rules at landfills violates the federal Clean Air Act.

Methane is a potent greenhouse gas that traps heat in the atmosphere, and municipal solid waste landfills are the third-largest source of human-made methane emissions in the U.S., according to the Environmental Protection Agency.

The Obama-era rule aimed at reducing air pollution, including methane, hasn’t been...