On the other hand, the deal could give Discover the additional backing it needs to compete with Visa and Mastercard, which some policymakers say have a duopoly on the credit card network market. American Express, the third-largest network, is both a payment network and an issuer, like Discover. More competition among payment networks could lead to lower swipe fees, which retailers could potentially pass along to cardholders. Bloomberg/Getty Images While it's unclear what changes are coming for cardholders, ...