LINCOLN, Neb. -- Seven states are opposing California's effort to enforce a state mandate that critics say discriminates against fuels produced outside California.

A federal judge ruled in December that California's Low Carbon Fuel Standard regulation violates the U.S. Constitution's commerce clause by discriminating against ethanol made in the Midwest. California has appealed and asked to be allowed to enforce the rule while its appeal is pending.

The regulation doesn't mandate using specific fuels, but assigns higher carbon scores to fuels made outside California. Groups challenging the regulation say it conflicts with federal standards and rules out fuels from other states.

Nebraska Attorney General Jon Bruning said Monday the regulation threatens $1.3 billion in annual ethanol sales from Nebraska alone. Also filing challenges are Iowa, Kansas, Michigan, Missouri, North Dakota and South Dakota.