Shares of Visa Inc. and MasterCard Inc. may rise as Senate Democrats struggle to muster votes for legislation that would help merchants cut credit card fees, a Morgan Stanley analyst said.

"The severity of the language in the proposed amendments, in their current form, increases the potential for a 60-vote approval requirement. We would be buyers on weakness," Morgan Stanley's Adam Frisch wrote Thursday in a note to clients after the companies' shares fell.

Three proposals by Senate Majority Whip Dick Durbin of Illinois could threaten more than $40 billion a year in interchange or "swipe" fees collected by card-issuing banks including Citigroup Inc. and JPMorgan Chase & Co.

MasterCard and Visa set the rates.

Shares of Visa and MasterCard fell on Wednesday after Sen. Patrick Leahy, a Vermont Democrat and chairman of the Judiciary Committee, backed an amendment to the financial industry overhaul legislation that would let merchants offer discounts for cash or a particular card brand and would allow retailers to set maximum or minimum transaction amounts.

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