The Justice Department's ongoing antitrust case against card network Visa may hinge on how the U.S. debit card market is defined. A company's ability to compete against Visa and others in the debit card processing market will be crucial in deciding how the litigation turns out, two attorneys who follow the industry said last week. "The expert phase will be a fight about market substitution – whether ACH, pay-by-bank, and newer payment networks belong in the same market as debit network services." The government argues that the company has carved out an illegal monopoly in the debit card market. The alleged monopolization in the debit case "runs through contracts rather than through a merger or a product design," Dickinson said in an email.