Three years later, no one is happy with a hotly contested rule intended to lower swipe fees on debit cards.

Banks were the original losers from the rule, which capped the fees they can charge merchants for debit card transactions. Now retailers are suing the Federal Reserve to lower the fees even more. And it appears that consumers haven’t reaped the benefits they were promised.

Created by the so-called Durbin Amendment to the 2010 Dodd-Frank financial reform law, the rule was the subject of intense lobbying between retailers and banks. The Federal Reserve finalized the regulation in June 2011, capping swipe fees at 21 cents per transaction, effectively halving the average fee.

But the rule’s supporters, including retailers, are trying to get the Supreme Court to force the Federal Reserve Board of Governors to lower the cap, arguing that it was set too high in response to lobbying from banks. Initially, the Fed had proposed a 12-cent cap.

Retailers, who lost at the appellate level in March, argue that the Fed’s fee cap is not a reasonable interpretation of the statute’s call for “reasonable and proportional” fees. They have a lot at stake: Swipe fees, officially called interchange fees, amount to roughly $16 billion annually, and retailers warn that they make up a rising share of their operating costs.

“Unfortunately, the board’s final rulemaking failed to sufficiently follow the text and purpose of the law,” Sen. Dick Durbin, D-Ill., wrote in a brief supporting the retailers’ challenge filed with the Supreme Court in September. “Because interchange fees are ultimately borne by consumers in the form of higher retail prices, consumers have suffered as a result,” argued Durbin, the amendment’s namesake.

Douglas Kantor, an attorney for the retailers, said he was “hopeful” that the Supreme Court would take up the case in January. If it is, he said, the case would hinge on whether the Fed had allowed banks’ costs unrelated to their debit card business to be factored into the calculation of what was a “reasonable” swipe fee.

Although the rule has helped consumers and merchants overall, Kantor said, the “results have been more mixed than it should have been” if the Fed had set a lower cap. In particular, the rule had the perverse effect of raising fees on transactions under $15 by effectively setting a floor on swipe fees as well as a cap. While the rule saved consumers $6 billion a year, they would have saved an additional $4 billion if the cap had been set at 12 cents, according to the report released by the Merchants Payments Coalition, an industry group.

Nevertheless, the rule has hurt banks by depriving them of $6 billion to $8 billion annually, according to Durbin Amendment critic Todd Zywicki. But Zywicki also argues that it has hurt consumers too. “The bottom-line conclusion is that by capping interchange fees on debit cards, the costs were initially transferred to banks, and then banks transferred on those costs to consumers,” said Zywicki, a law professor at George Mason University School of Law.

“The tragedy here is it’s not just Citigroup on one side and Walmart on the other,” he said. He argued that the rule has “pushed low-income households out of the mainstream banking system” by leading large banks to offer fewer low-cost checking accounts.

Because banks with less than $10 billion in assets were exempted from the rule, Zywicki says that he and his co-authors were able to discern the rule’s impact by comparing large and small banks in a recent study published by the International Center for Law and Economics, a think tank that has received funding from MasterCard.

At big banks, the number of free checking accounts has fallen and the average required balance for those accounts has climbed, they found, while small banks not subject to the rule have increased the availability of free accounts.

Furthermore, they noted, consumers have increasingly turned to credit cards and prepaid cards to avoid fees, and the overall number of unbanked Americans has risen, according to the Federal Deposit Insurance Corporation.

Kantor challenged those conclusions, noting that the study drew on some bank-fee data from before the regulations went into effect. The “findings on banking fees are actually not relevant to the question of the impact of the reforms themselves and show that other things were at work,” Kantor argued. In particular, he noted that the fallout from the 2008 financial crisis also likely prompted banks to pull back on their checking account offerings.