Shares of Visa and Mastercard declined following the U.S. Senate's passage of the GENIUS Act, which establishes federal oversight for stablecoins. This legislation could facilitate the adoption of lower-cost, stablecoin-based payment systems by companies like Amazon and Stripe, challenging traditional card networks. Experts warn that Visa and Mastercard may experience slower growth and reduced fees as these new digital currency solutions gain traction. The move signals a significant shift in the payment industry landscape.