Two members of the Senate Banking Committee wrote Federal Reserve Chairman Ben Bernanke asking him to “create a workable small issuer exemption,” in the debit interchange rule that the Fed is writing.

Sens. Michael Bennet (D-Colo.) and Kay Hagan, (D-N.C.), both of whom voted for the amendment to the financial overhaul bill that gave the Fed the power to regulate interchange fees, wrote that they are “very concerned that the rule might prevent small issuers from benefiting from the [small issuer] exemption set forth in the Act.”

They also urged the Fed to take into account the fraud prevention costs that debit card providers must absorb.

The senators expressed concern that consumers could lose access to free checking or other services if financial institutions are hurt by reduced fee income.

The Fed’s comment period on the proposed rule ended Tuesday. Unless Congress acts to change the requirements set forth in the financial overhaul bill, the rule must be approved by April 21 and in effect by July 21.