THE multibillion dollar United States credit card industry feels like the kid at the bottom of a schoolyard "pile on."President Bush has called for them to cut the interest rate they charge customers. The Senate passed a bill last week putting a 14 percent cap on credit card rates, down from an average of nearly 20 percent now. A number of similar bills have been introduced in the House. Mr. Bush has promised to veto a mandatory cap. Nonetheless, the Senate's move was regarded as one factor behind the recent plunge in stock prices. The dispute involves the wide spread between the rates that banks charge on cards and what banks pay on deposits, about 5.5 percent for one-year certificates of deposit. Even before the current brouhaha over interest rates, it was clear that the industry was questioning what new direction it should take, given the broad availability of credit cards. "There's just a lot of plastic out there now," says Robert McKinley, president of RAM Research USA in Frederick, Md., a market research firm. Currently, there are some 265 million bank credit cards outstanding, Mr. McKinley says. Counting retail cards, such as gasoline and department store charge cards, there are close to 1 billion cards in circulation. Most adult Americans now carry one or more cards. Not surprisingly, delinquency rates have slowly risen for some bank cards with the recent economic downturn - but perhaps none as sharply as for American Express Company. McKinley says that while there is still adequate rate competition (some banks offer cards with rates as low as 9 to 10 percent,) the major credit card issuers will feel increasing pressure to voluntarily reduce their own rates. Nor, he says, is it totally inconceivable that Washington might actually enact some form of ceiling. Were that to happen, he says, the cap would probably be nearer 18 percent than 14 percent - and probably linked by some formula to general interest rates. At present, eight of the 10 largest issuers of bank cards have standard interest rates of 19 percent or more. For some big banks, such as Citicorp or Chase Manhattan, reductions could hurt, given their high rates. Ironically, American Express Company might benefit from a push towards lower rates, or even a rate ceiling, according to analysts such as Michael Lewis of Dean Witter & Co. "The impact on American Express would be largely positive," he says, since a rate reduction or cap would probably hurt competitors of American Express more than American Express itself. Moreover, there would probably be less competition for AmEx as commercial banks issued fewer new cards. Over time, American Express "might well benefit from rate reduction," says Joan Solotar, an analyst with Donaldson, Lufkin & Jenrette. AmEx stunned the financial community in early October when it announced substantial defaults on its Optima card - enough, in fact, to require a $265 million after-tax charge to offset bad debts and a restructuring of the company's Travel Related Services division, its most important division. Retail outlets are also increasingly restive over the relatively high fees they are forced to pay on AmEx charges. American Express has some 37 million cards outstanding. Its green, gold, and platinum charge cards tend to be well-regarded money-earners within the credit-card industry, with a low delinquency rate. The problem for the company involves the 3 million Optima cards outstanding; the Optima card was launched in 1987. The Optima card, issued by the company's Centurion Bank unit, has a revolving credit line, similar to Visa and Master Card. (In the case of the company's green and other cards, the balance is usually paid in full each month.) In early October, AmEx said that charge-offs on delinquent Optima cards had soared to about 8 percent, roughly double the delinquency rate for the average bank card. If interest rates on cards start to come down, AmEx might not take as much of a hit as other card companies; Optima's rates are already low, typically at 16.25 percent.