Airlines, labor unions, and manufacturers have written to senators warning that a bill about credit cards being proposed by senators could damage the aviation industry by weakening airline loyalty programs.

United Airlines, American Airlines, Southwest Airlines, Airbus, and Boeing are among the signatories of Monday's letter.

The campaign, led by the trade group Airlines for America, takes aim at legislation proposed by Senators Roger Marshall and Dick Durbin.

Their amendment to the crypto-focused GENIUS Act hopes to expand competition for credit card providers and lower swipe fees.

It's been supported by retailers, including the National Retail Federation, but has faced backlash from financial institutions.

Credit cards also play a significant role for airlines. "A lot of people call airlines credit card companies with wings," TJ Dunn, a points guru and editor in chief at the Prince of Travel, previously told Business Insider.

Co-branded credit cards are a cash cow for many airlines, allowing customers to build up frequent-flier points or miles.

Monday's letter cites research from Airlines for America that says over 31 million Americans have airline travel reward cards, generating around $25 billion in economic activity in 2023.

It adds that 57% of all frequent flyer miles or points issued in 2023 were generated by airline credit card use.

"Americans value and enjoy credit card rewards programs because they reward consumers for dollars that they would be spending no matter what," the letter says. "Many may be unpleasantly surprised if Congress disrupts those programs."

It warns that the Durbin-Marshall amendment could see airlines stop offering rewards credit cards.

Unions, including those representing pilots, flight attendants, and Boeing workers, have signed the letter, too. That's because it says the effect on loyalty programs would result in fewer flights and therefore "a contraction in airline activity and jobs."

It adds that lost revenue means airlines would struggle to meet existing commitments to their workers, and "certainly complicate existing or future collective bargaining negotiations."

The letter also says airlines would have less money to spend on buying new planes.

Durbin has said that the legislation would save merchants and consumers an estimated $15 billion each year, while businesses pay more than $100 billion in swipe fees annually.