Key TakeawaysVisa shares fell Tuesday as the U.S. Department of Justice (DOJ) filed an antitrust lawsuit against the payment processor.The DOJ accused the company of using monopolistic tactics to retain its top spot in the debit card market.The suit is the latest in a string of recent antitrust moves from the DOJ and Federal Trade Commission. Visa (V) shares fell Tuesday as the U.S. Department of Justice (DOJ) sued the payment processing giant over allegedly monopolistic behavior to protect its dominant pos ...