SHOWDOWN: Conventional wisdom says that an incumbent with a healthy lead can only lose ground by debating. . . . Nonetheless, Lt. Gov. Leo T. McCarthy has agreed to spar with his Republican challenger, state Sen. Marian Bergeson--on Bergeson’s home turf, no less. Their only scheduled face-to-face meeting will take place Oct. 3 at the Huntington Beach studio of KOCE. “The person who thinks they’re going to win” should agree to the debate, says McCarthy’s campaign manager.