A federal judge has decided not to let environmental groups join the legal battle between North Dakota and Minnesota over coal power plants.

The ruling Friday by U.S. Magistrate Judge Steven Rau said 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. In a concession to the groups, Rau said they may later ask permission to offer a "friend-of-the-court" submission.

North Dakota's attorney general, joined by three utilities, two coal companies and a lignite trade group, are suing to invalidate a 2007 Minnesota law restricting imports of electricity from new coal-fired power plants in other states. The law, known as the Next Generation Energy Act, discourages new sources of carbon dioxide, a greenhouse gas linked to global warming.

North Dakota, where lignite coal is mined, claims the law violates the U.S. Constitution's commerce clause and federal law.