A marketing battle over 1 million frequent flier cardholders has landed in court.

Late last week Delta Air Lines Inc.'s Northwest Airlines Inc. sued U.S. Bancorp to prevent it from converting the cardholders from the Northwest WorldPerks reward program to a multiple-airline program.

In a complaint filed in the U.S. District Court for Minnesota, Northwest said that the plan U.S. Bancorp announced this month to switch cards to its new FlexPerks program would breach the issuer's contract with the airline.

The contract does not allow U.S. Bancorp "to award anything other than WorldPerks miles to a customer," according to the suit, which asked the court to issue a temporary restraining order.

U.S. Bancorp lost a cobranding contract with Northwest last year after the carrier was acquired by Delta, which has a longtime cobranding partner in American Express Co.

On Monday, Amex unveiled a multimedia advertising campaign meant to persuade U.S. Bancorp cardholders to switch to its Delta SkyMiles cards.

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