BOSTON — Congress appears poised to tighten restrictions on the fees that merchants pay when customers swipe debit cards at cash registers.

An amendment that cleared the Senate late Thursday is a loss for card payment networks like Visa and MasterCard — whose stocks tumbled yesterday — as well as major banks that issue cards.

But even if the amendment becomes law, it's unclear whether savings from reductions in debit card swipe fees would trickle down to consumers in the form of lower prices, or would largely be kept by merchants.

Swipe fees, formally called interchange fees, are charges that a merchant's bank pays the issuer of a customer's card for each electronic transaction.

The proposed change isn't likely to produce a windfall for shoppers, according to Scott Valentin, an FBR Capital Markets analyst who follows financial company stocks.

Valentin said he expects "little, if any, of any cost savings gained by merchants to be passed on to consumers."

Beyond the cash register, lower swipe fees also could affect increasingly popular debit card rewards programs, where customers rack up points for goodies like airline tickets or the latest iPod model.

Valentin and another analyst said those programs could disappear, because banks rely on revenue from interchange fees to help cover the programs' reward costs.

The programs "would likely diminish or go away once debit interchange revenues are reduced," wrote Credit Suisse analyst Moshe Orenbuch.

However, a merchant who has sued card companies and banks over swipe fees says savings from fee cuts would ultimately reach consumers.

"It will transfer billions of dollars from banks to American families and small businesses," said Mitch Goldstone, CEO of Scanmyphotos.com, an Irvine, Calif. e-commerce and retail photo business.

If merchants pay lower fees on customers' debit payments, those that don't respond with price cuts will lose cost-conscious customers, said Goldstone.

When businesses accept major credit cards they sign agreements with the card companies to pay a percentage of each transaction, usually about 2 to 3 percent. Fees are set in negotiations among the various parties, although merchants say terms are largely dictated by card companies and banks.

The amendment that cleared the Senate involves interchange fees on debit cards, but not credit cards. Debit cards are being used more as customers have become less willing to borrow during the recession.

Still, swipe fees from credit card transactions make up a bigger portion of bank profits than fees from debit transactions.

A version of the legislation that cleared the House in December didn't include restrictions on interchange fees.

So Thursday's Senate passage of an amendment by Democratic Whip Dick Durbin of Illinois came as a surprise. Seventeen Republicans were among the 64 senators voting in favor.

The proposal would give the Federal Reserve authority over interchange fees on debit cards issued by banks with more than $10 billion in assets. Most debit cards are issued by large national banks with assets above that threshold.

The legislation directs the Fed to ensure that fees are "reasonable and proportional" to actual costs, and implement rules within a year of the law's passage. Card issuers wouldn't be barred from making a profit from the transactions.

The proposal now will be folded into negotiations between House and Senate leaders to resolve differences between each chamber's version of the financial overhaul bill.

Amid a broader market decline, shares of Visa Inc. dropped $8.47, or nearly 10 percent, to close at $77.26, while shares of its biggest rival, Mastercard Inc., fell $19.86, or 8.6 percent, to $212.45.

American Express Co. doesn't offer debit cards but saw its stock fall anyway, losing $2.17, or 5 percent, to $40.64.

Losses were smaller for shares of major card-issuing banks. Bank of America fell 53 cents, or 3.1 percent to $16.34; Citigroup slipped 11 cents, or 2.7 percent, to $3.98; and JPMorgan Chase fell 92 cents, or about 2.3 percent, to $39.89.

Orenbuch, the Credit Suisse analyst, estimates a 20 percent reduction of debit interchange fees could result in a 2 percent to 4 percent drop in revenue for Visa and Mastercard.

But Janney Capital Markets analyst Thomas McCrohan said Visa and Mastercard have other pricing options on their card services to make up for the revenue loss.

The financial industry is fighting the Senate proposal. The Electronic Payments Coalition, whose membership includes Visa and Mastercard as well as major banks such as Bank of America and Citi, said the Durbin amendment would leave consumers "holding the bag for giant retailers who don't want to pay their fair share" for card transactions.

A statement from the American Bankers Association said the measure would hurt banks of all sizes "because it directly inserts the government into a price-fixing role and mandates competitive inequities in the marketplace."

But Goldstone, the photo business owner who's suing banks and card companies, said the tighter swipe fee restrictions would foster greater competition. Small businesses, he says, are largely at the mercy of Visa and Mastercard over the fees — his company's third-biggest fixed expense after rent and personnel.

"The challenge as a retailer is that I have to accept it," he says. "Every e-commerce business from Amazon.com on down must accept Visa and Mastercard, or they're out of business."