Credit cards are reshaping airline strategies, becoming the sharpest competitive weapon in carriers' fight for travelers. The 3.4 inch-by-2.1 inch plastic is increasingly essential to airlines' bottom lines and central to decisions from lounge locations to route planning.

Delta now restricts lounge access for customers buying cheapest business-class tickets unless they hold the right credit card. Southwest is leasing airport space for new lounges to create perks for its cobranded cards.

At O'Hare International Airport, American Airlines is fighting to retain gate access in its terminal because the Chicago hub fuels its credit-card loyalty program. Credit-card partnerships now drive nearly every strategic move airlines make.