Wal-Mart store in Secaucus, New Jersey - Jin Lee-Bloomberg via Getty Images
How do you get labor unions and banks on the same side? Just mention Wal-Mart (WMT).

Unions have been hammering away at the nonunion discount mega-chain over worker compensation, hours, its stance on minimum wage increases and hiring practices for much of the past year. Yet it's Wal-Mart's "backdoor" foray into the banking industry and its sales of prepaid cards that brought banks into the battle against the retailer.

The Federal Advisory Council, a group of banks including PNC Financial Services Group (PNC) and BB&T (BBT), told the Federal Reserve at a meeting in December that Wal-Mart's sales of prepaid cards warranted greater federal oversight, according to meeting minutes obtained by Bloomberg and cited in a story published Wednesday.

The council urged the Fed to consider limiting payment-related services like prepaid cards to "regulated banking institutions" or to step up regulation of that business. It also suggested the government unleash the Consumer Financial Protection Bureau, created by the 2010 Dodd-Frank law, on nonbank companies like Wal-Mart that provide financial services.

Banks are becoming a fairly regular opponent of Wal-Mart and haven't been shy about teaming up with some of the chain's other enemies to block expansion onto their turf. In 2005, banks joined labor unions and community groups to oppose Wal-Mart's application to open a limited-service bank based in Utah. The retailer eventually abandoned that plan.

In 2011, banks asked the consumer bureau to consider Wal-Mart as a "larger participant" in financial services, which would lead to direct oversight.

As if to taunt the banks, Wal-Mart immediately followed up the council's request by partnering with American Express (AXP) and offering a prepaid card called Bluebird. While similar to a card offered by JPMorgan Chase (JPM), Bluebird raised eyebrows among banks by detaching itself from banks entirely. Wal-Mart says its partners like American Express are already regulated, but the banks aren't having any of it.

The strongest argument the banks make is that Wal-Mart's plan is just a big ploy to get around interchange, or card swipe, fees by hoarding all the fee money for itself. In 2010 and 2011, retailers including Wal-Mart, Target (TGT) and Home Depot (HD) successfully fought the banks to get the Fed to mandate limits on swipe fees.

Prepaid cards, however, are exempt from those limits. The banks argue that the exemption "appears to permit Wal-Mart, a strong proponent of lower debit interchange rates, to benefit indirectly from the very thing it opposed: unregulated interchange."

With customers knocking Wal-Mart in increasing numbers for its empty shelves and long checkout lines, the retailer's banking strategy is attracting more enemies than business.

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