Fed said data it collected show all networks provided a higher average interchange fee to exempt issuers than non-exempt issuers in 2012.
Debit card issuers with more than $10 billion in assets can charge a one-cent fraud-prevention adjustment as of Oct. 1, but not most credit unions.
Credit unions have argued before U.S. District Judge Richard Leon in defense of the Federal Reserve's existing Durbin amendment regulation.
House and Senate conferees will tackle the issues of greatest interest to credit unions in the financial overhaul when they meet later today.
The Competitive Enterprise Institute, a Washington think tank with a conservative reputation,