PHOENIX — Mesa Air Group Inc. filed a federal lawsuit Monday against Delta Air Lines Inc. to keep Mesa's contract flying agreement intact, a lawyer for Mesa said.

Mesa General Counsel Brian Gillman said the lawsuit filed in Atlanta was in response to Delta's announcement last week that it plans to end the contract.

"We're asking the court require Delta to honor the contract as long as Mesa carries out its end of the bargain," Gillman said.

Losing Delta's contract would be a major blow to the Phoenix-based carrier. The contract was worth $20 million in revenue per month last year and comprised 21 percent of the carrier's total passenger revenue for the three-month period that ended Dec. 31.

Delta noted last week that Mesa's performance was lacking. Specifically, Mesa said, the Atlanta-based carrier criticized the number of ERJ-145 Delta Connection flights Mesa's subsidiary, Freedom Airlines, completed from September 2007 through February.

But Mesa said there's nothing wrong with the regional carrier's performance. Mesa said Delta asked it to cut flights and now is blaming the Phoenix carrier for the low completion rate.

Mesa contends that Delta is simply trying to end its contract with Mesa in an effort to cut flights and boost profits.

"Now that Delta wants out of the contract, it has contrived a new way of calculating completion rate that counts those cancellations against Mesa," Gillman said. "This new interpretation is obviously unreasonable, because it would give Delta the ability to declare default whenever it wanted just by canceling enough of Freedom's flights."

Delta spokeswoman Chris Kelly wouldn't comment about the lawsuit except to say that Delta would "vigorously defend" itself.

"Delta has notified Freedom that we will be canceling the contract because operational performance has fallen below minimum levels required under the contract," Kelly said. "We expect the pull-down to be orderly and have minimal customer impact."

Mesa operates 182 aircraft to 157 cities and employs about 5,000 people. It operates as Delta Connection, US Airways Express and United Express under contractual agreements with Delta Air Lines, US Airways and United Airlines, respectively. In June 2006, Mesa launched go!, an inter-island service in Hawaii.

Mesa Airlines, a commercial carrier owned by Mesa Air Group, had flights to Cedar City, Moab and Vernal until last May, when it discontinued those routes, saying they were not profitable.

Contributing: Laura Hancock, Deseret Morning News.