Supporters of the Tester-Corker amendment were concerned about Sen. Joseph Lieberman (D-Conn.), who supports the Tester amendment but may not vote because he is an Orthodox Jew and today is the Jewish holiday of Shavout. A spokesman for Lieberman said he would check on whether the senator plans to cast votes today.

However, whip counts can be unreliable since senators sometimes switch at the last minute in order to be with the winning side.

Both CUNA and NAFCU have had members on Capitol Hill in the past few days lobbying senators and staffs. This supplements the extensive grass roots efforts by both groups both individually and through the Electronic Payments Coalition.

That group has also run an extensive advertising campaign, which was matched by similar efforts at the retailers. These ads have appeared both in the Washington, D.C. area and in the home states of key senators.

The amendment calls for a six-month study by the NCUA and other regulators to consider: the fixed and incremental costs to the financial institutions of the new rules; whether the rules will adversely affect consumers, and whether the small issuer exemption is feasible.

If the Fed and at least one other regulatory agency make any of the above determinations, then the Fed must rewrite debit interchange rules within six months. Otherwise, the Fed rule would move forward.

Senate Majority Whip Dick Durbin (D-Ill.) joked about how the fight on interchange fees has been a boon for lobbyists on both sides.

"A friend of mine who is a lobbyist downtown in Washington said, 'Durbin, praise the Lord. Come up with some more ideas. This is a full-employment amendment. Everybody who is a lobbyist in Washington is working on this amendment. We just love you to pieces,"' Durbin said on the Senate floor Tuesday.