Visa (NYSE:V ) stock is dipping 3.1% and Mastercard (NYSE:MA ) stock is slipping 1.4% after the Wall Street Journal reported Wednesday that two U.S. senators are preparing legislation aimed at creating more competition among credit card networks.

Senator Dick Durbin (D-IL) and Senator Roger Marshall (R-KS) may introduce the bill as soon as this week. The bill is similar to a debit-card rule Durbin had backed more than 10 years ago. The amendment, part of the 2010 Dodd-Frank Act, gives merchants the ability to choose from at least two unaffiliated debit card networks when processing transactions.

Representative Peter Welch (D-VT) is expected to serve as lead Democrat on a similar House bill, the WSJ said, citing a person familiar with the matter.

Most bank-issued credit cards are affiliated with either Visa (V) or Mastercard (MA). When a consumer pays using a credit card, the merchant is required to pay card-related fees, including an interchange fee to the card issuer and a network fee to either Mastercard (MA) or Visa (V).

Merchants, including stores, restaurants, and hotels, that accept the cards have long complained about the fees. The networks, though, say the fees are needed to pay for innovations and fraud prevention.

The transaction usually has to take place on the network affiliated with the card. The bill, though, would require that merchants would often have the right to route payments through an unaffiliated network, the WSJ said.

American Express (NYSE:AXP ) is off 0.8% in Wednesday afternoon trading and Discover Financial (NYSE:DFS ) is rising 0.9% .

In February, Amazon (AMZN) and Visa (V) reached an agreement over credit card swipe fees. At one point, Amazon threatened to stop accepting Visa credit cards that were issued in the U.K.

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