MINNEAPOLIS — A federal judge won’t let environmental groups join the legal battle between North Dakota and Minnesota over coal power plants.

U.S. Magistrate Judge Steven Rau ruled Friday that the Minnesota attorney general is able to defend the lawsuit, and the case “will not be enhanced in any significant way” by adding environmental interests as parties.

Seven environmental groups wanted to join the fight, including the Minnesota Center for Environmental Advocacy, a St. Paul nonprofit.

North Dakota’s attorney general, joined by three utilities, two coal companies and a lignite trade group, is suing to invalidate a 2007 Minnesota law restricting imports of electricity from new coal-fired power plants in other states.