The man who could arguably be called the catalyst for the Justice Department's antitrust lawsuit against Visa and MasterCard, Harvey Golub of American Express Co., took the stand for the government Wednesday and argued that if banks were allowed to work with his company, consumers who use credit cards could have lower interest rates.

Mr. Golub, chairman and chief executive officer of American Express, is not exactly the government's star witness - his deputy and hand-picked successor, Kenneth I. Chenault, who testified last week, had already covered the points the government was trying to prove, and seemed a more charismatic raconteur. But it was the soft-spoken Mr. Golub who opened Pandora's box four years ago when, in a keynote speech at a major credit card conference, he invited banks to become partners with American Express - a move that riled some of his colleagues, he revealed Wednesday.

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