NEW YORK (CNNMoney.com) -- Shares of Visa and MasterCard tumbled Friday amid fears that a new rule proposed by Congress would take a severe bite out of the money they earn when consumers use their debit cards.
Prompting the sell-off was news that the Senate adopted an amendment late Thursday that would effectively limit what the two card network operators can charge through so-called "interchange" fees.
Currently, Visa and MasterCard earn a fee every time a consumer swipes their credit or debit card for processing a payment, which gets passed onto both consumers and retailers. In 2007 alone, the pair earned $35 billion from interchange fees, according to recent government estimates.
If the proposal eventually becomes law, Visa and Mastercard would be limited to charging a more "reasonable" fee, as determined by the Federal Reserve.
Visa (V, Fortune 500) stock fell more than 10% Friday, while shares of Purchase, N.Y.-based rival Mastercard (MA, Fortune 500) finished about 9% lower.
Some analysts who track the pair wondered whether investors were overreacting however.
"Even in this form, the amendment seems a little tamer than what was initially imagined," said Zaineb Bokhari, an equity analyst with Standard & Poor's who continued to recommend the stocks depsite Thursday's announcement.
For starters, the bill only affects the two firms' debit card transactions, which is not the bulk of their business. The amendment would not impose any restrictions on the fees charged on credit card transactions.
At the same time, both Visa and MasterCard will continue to enjoy significant growth in the international part of their business, namely on fees generated when travelers use their credit cards overseas.
And while the amendment made it into the Senate's version of the reform bill, it remains to be seen whether it will actually make it to President Obama's desk.
Others noted that card issuers, namely the banks, stood to suffer even more as a result of the amendment.
"We expect issuers ... to bear the majority of the financial brunt of any potential reduction in interchange fees," wrote FBR Capital Markets' analyst Scott Valentin in a note to clients Friday.
Lenders typically collect the bulk of the fees that are charged to conduct a credit or debit card transaction, which can range anywhere from 1% to 3% of every purchase.
Like network operators Visa and Mastercard, banks would be be required to abide by the new interchange rules, according to the proposal
Bank of America (BAC, Fortune 500), JPMorgan Chase (JPM, Fortune 500) and Wells Fargo (WFC, Fortune 500) each finished more than 2% lower Friday, falling along with the broader market.
Discover (DFS, Fortune 500) and American Express (AXP, Fortune 500), two companies which both offer cards to customers and have their own card networks, were also sharply lower, falling about 4% and 5% respectively.
The amendment however, would likely hit only the biggest of the nation's banks. Lenders and credit unions with $10 billion in assets or less would be allowed to charge the same rates that they currently do. 
| Index | Last | Change | % Change |
|---|---|---|---|
| Dow | 32,627.97 | -234.33 | -0.71% |
| Nasdaq | 13,215.24 | 99.07 | 0.76% |
| S&P 500 | 3,913.10 | -2.36 | -0.06% |
| Treasuries | 1.73 | 0.00 | 0.12% |
| Company | Price | Change | % Change |
|---|---|---|---|
| Ford Motor Co | 8.29 | 0.05 | 0.61% |
| Advanced Micro Devic... | 54.59 | 0.70 | 1.30% |
| Cisco Systems Inc | 47.49 | -2.44 | -4.89% |
| General Electric Co | 13.00 | -0.16 | -1.22% |
| Kraft Heinz Co | 27.84 | -2.20 | -7.32% |
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