The shares dropped $1.97, or 4.5 percent, the most in more than two months, to $42.04 at 12:08 p.m. in New York Stock Exchange composite trading. AmEx had climbed 8.6 percent this year through yesterday.

The Federal Reserve last week proposed cutting the “swipe” fees, or interchange, that merchants pay to accept debit cards by up to 84 percent. That will give merchants more incentive to steer consumers away from New York-based AmEx’s higher-cost credit and charge cards, said Chris Brendler, a Stifel analyst, in a note to clients today. He changed his recommendation for the stock to “hold” from “buy.”

“We are increasingly convinced that credit interchange will inevitably be the next target,” he wrote. “The now- significant disparity in payments’ costs greatly increase the risk.”

Visa Inc., the world’s biggest payments network, plunged 17 percent last week, and No. 2 MasterCard Inc. fell 13 percent, as the Fed proposed capping debit interchange at 12 cents per transaction. AmEx, which doesn’t issue debit cards, charges retailers who accept its credit and charge cards an average of 2.56 percent of the purchase price, the lender said in its third-quarter financial supplement.

American Express, the biggest credit-card issuer by purchases, is fighting a federal antitrust lawsuit that claims the company’s contracts unfairly prohibit retailers from steering customers to cheaper card brands. Visa, based in San Francisco, and Purchase, New York-based MasterCard said in October they agreed to settle the Justice Department complaint.