A U.S. senator representing the country’s leading car-building state has called on the top U.S. trade official to demand Russia stop what she calls unfair trade practices against American-made cars.

Democrat Debbie Stabenow of Michigan wrote in a letter to U.S. Trade Representative Ron Kirk Wednesday that Russia’s position must not be tolerated. She says Kirk must make it clear to Russia that dropping the barriers must be part of Russian negotiations to join the World Trade Organization.

Stabenow says Moscow requires each Russian carmaker to build 300,000 vehicles a year if it wants to avoid high tariffs on imported car parts.

She says this hurts U.S. car manufacturers and notes that such quotas are outlawed under WTO rules.

Moscow must still negotiate the final terms of its entry into the World Trade Organization. Russia is the largest economy outside the WTO and hopes to end its 18-year bid to join the world trade body by the end of 2011.

The United States approved Russia’s WTO membership in October.