More credit card rules could hurt Visa, MasterCard

* Visa and MasterCard shares seen under pressure

* Pricing power at risk as banks struggle with losses

* Visa and MasterCard deny impact from regulations

By Juan Lagorio

NEW YORK, June 22 (Reuters) - Visa Inc (V.N) and MasterCard Inc (MA.N), the world's largest payment networks, could face lower revenue and pressure on their stock prices amid a push for increased U.S. regulation of credit cards.

Specifically, Congress is mulling regulations on interchange rates -- fees retailers and merchants have to pay to banks that issue credit cards.

Most immediately, that would affect the banks that collect those fees. But some investors and analysts are concerned the banks -- already battered by credit losses and toxic assets -- could try to share the haircut with Visa and MasterCard.

"Longer term, I think it is a real risk for the model, but this could impact their stocks right now," said Donald Fandetti, an analyst at Citigroup.