The WorldPerks Visa card may be just a piece of plastic. But to Minneapolis-based U.S. Bank, it's worth tens of millions of dollars, and that big bundle of money is at risk with the proposed Northwest Airlines merger with Delta Air Lines.

With an estimated 800,000 to 1 million WorldPerks cards in circulation, the annual fees alone come to $52 million to $65 million. Add tens of millions of dollars more in interest on outstanding balances and in merchant fees.

Many credit-card and airline experts see the WorldPerks credit card as a long shot to survive the transition to a new Delta, which for years has had American Express and its SkyMiles card as its partner.

Delta may use the merger as an occasion to set U.S. Bank and Amex against each other in a bidding war that could result in higher costs for the winner -- and for frequent fliers, said R.K. Hammer, chief executive of a Los Angeles bank card advisory firm that bears his name.

"Cinch up your seat belts, you're going to get squeezed," he said.

But nothing is likely to happen this year or perhaps even for months after the airlines merge, if they get government approval.

"There's a long way to go until the merger is complete, but we are comfortable that the contracts we have in place with Delta will continue our partnership," said Desiree Fish, an Amex spokesperson.

U.S. Bank also was eager to point out that no decision has been made on the future of the WorldPerks program.

"It's very early," said John Owens, U.S. Bank senior vice president of retail payment solutions. "There are a number of scenarios. It's fair to say it's a significant piece of business. We believe we're well positioned to keep up with new opportunities. But to say what they are is premature."

When U.S. Bank wrested away Northwest's frequent-flier affinity card business from Bank One in 1994, Bank One attempted to retain customers by promising fliers that they could cash in card credits for flights on a number of airlines.

"There may be some attempt to steer consumers from one card to another," said Michael Englund, chief economist at Action Economics in Boulder, Colo. "But I'd be surprised if people were forced to switch to Amex."

Amex has an estimated 3.7 million SkyMiles customers, who pony up an estimated $300 million-plus in annual fees alone.

That arithmetic alone -- 3.7 million users compared with 1 million or fewer WorldPerks users -- favors Amex. But the SkyMiles card issuer has more going for it than that, said Randy Petersen, editor and publisher of InsideFlyer magazine.

Which is simpler, Peterson asked, to transfer WorldPerks customers to the SkyMiles card or to port nearly five times that number of SkyMiles customers to U.S. Bank?

"Amex goes in big," Petersen said. "I have no doubt they'll win this one."

Amex also has a history of coming to Delta's aid when the company needs it most.

Nearing the steps of bankruptcy court in 2004, Delta got a big lift from Amex.

New York-based Amex prepaid $500 million for frequent-flier miles to be dispensed through its SkyMiles card. It also stepped up to provide a $100 million loan.

Those gestures alone could tip the balance in deciding which card survives the merger, Petersen said.

Hammer, the credit-card analyst, expects a decision to be made by Delta sooner rather than later.

The questions Delta executives will ask U.S. Bank and Amex, he said, will be simple: "What will you do for our passengers? What are you going to do for us?"

Mike Meyers • 612-673-1746