Aiming to pump up its political muscle, Visa U.S.A. has hired a veteran lobbyist, Mark MacCarthy, whose roots are in Democratic politics. Mr. MacCarthy, who joined the credit card association last week as senior vice president for public policy, will work alongside longtime Visa lobbyist Lamar Smith, who has strong ties to Republican lawmakers, according to Visa spokesman Kelly Presta. Mr. MacCarthy had worked for the Wexler Group, a Washington government-affairs firm founded by Anne Wexler, a former aide to President Jimmy Carter. At the Wexler Group, he worked with Visa for four years, primarily on electronic commerce and privacy issues. Visa is under extraordinary legal and political …