Visa Inc. (V) and Mastercard Inc. (MA) are having the kind of week that makes investors nervous. Both payment giants are tracking toward their worst five-day stretch since the April tariff chaos, with shares down roughly 4% as regulatory threats pile up. Here's what's rattling the market.

A Legislative Threat to the Duopoly

The selling pressure kicked into gear after President Donald Trump endorsed the Credit Card Competition Act, a piece of legislation that would require big banks to route transactions over at least two unaffiliated networks. That might sound technical, but it's actually a direct shot at the cozy arrangement Visa and Mastercard have enjoyed for years.

Right now, the two companies handle about 84% of U.S. credit card volume, anchoring an ecosystem that generates an estimated $223 billion in yearly revenue. The Competition Act would crack that open, forcing transactions onto rival networks and potentially siphoning off volume.

Goldman Sachs ran the numbers and found that even a 5% shift in payment volume could shave about 3% off Visa's earnings and 1% from Mastercard's. And that's the optimistic scenario. If political pressure pushes interchange fees down toward the much lower rates common internationally, the damage could be significantly worse.

The threat extends beyond just transaction volume. It could slow growth in Visa's dominant U.S. debit and credit franchise and hit Mastercard's higher-margin cross-border and data-services segments, which have been key growth drivers.

Trump's Rate Cap Demand Adds Another Layer

As if the Competition Act weren't enough, Trump has piled on by demanding that card companies cap interest rates at 10% by Jan. 20. He's called current APRs of 20% to 30% "abuse" and warned that failure to comply would be treated as "a violation of the law" with "severe" consequences.

Now, here's the thing: Visa and Mastercard don't actually lend money to consumers. They're networks, not banks. But investors are reading Trump's aggressive rhetoric as a signal that the entire credit card economic model is under scrutiny, from rewards programs to swipe fees.

How This Hits the Fee Engines

Understanding why this matters requires understanding how Visa and Mastercard make money. They sit in the middle of nearly every card transaction, charging issuers and merchants network and assessment fees. Those fees are ultimately supported by the profitability of credit card lending.

If regulation squeezes issuers' revenue through lower APRs and forces routing options that bypass Visa and Mastercard rails, the dominoes start falling. Banks will demand lower network fees. Merchants will gain leverage to negotiate down swipe costs. Volume will migrate to cheaper alternative networks.

That combination threatens the high-margin, recurring revenue streams that make these businesses so profitable—whether it's Visa's massive U.S. debit and credit operation or Mastercard's premium cross-border and data-services segments.

Where the Stocks Stand

Mastercard, Visa Price Action: According to data from Benzinga Pro, Mastercard is trading near $539, off 4.33% over five days, while Visa hovers around $328, down 4.21% over the same period.