The union for mechanics at Frontier Airlines is going to court over plans to shift their work to Milwaukee.

Indianapolis-based Republic Airways Holdings Inc. bought Frontier last year, and wants to move Frontier's maintenance operation from Denver to Milwaukee, where work on the company's other jets is already done.

But the International Brotherhood of Teamsters, which represents Frontier mechanics, said in a court filing that their contract guarantees that they do that maintenance work. They said Republic is offering to transfer mechanics to Milwaukee, but they would no longer be in the union.

Republic has not yet responded to the request for a court order enforcing their contract. The request was filed late Wednesday in U.S. District Court in Milwaukee.

According to documents filed by the Teamsters, Frontier mechanics who move to Milwaukee have been told they'll no longer be covered by the union.

Republic spokesman Carlo Bertolini declined to comment directly on the Teamsters court papers. He said that Republic was aiming to have the new maintenance operation running in Milwaukee this month, but can't do it yet because only 35 Frontier mechanics agreed to make the move. Republic needs about 125.

Republic bought Milwaukee-based Midwest Airlines last year before buying Denver-based Frontier. Republic also does regional flying for hire by major airlines.