<p>ARTHUR Schlesinger Jr., a lifelong Democratic stalwart, says there is only one Democratic candidate for president he wouldn't vote for - Sen. Joe Lieberman. "He's the most sanctimonious member of the U.S. Congress," the eminent historian tells Avenue columnist Richard Turley in a wide-ranging conversation. Schlesinger says that on the 40th anniversary of the assassination of John F. Kennedy, "I've always been agnostic about the theory of the possibility of a conspiracy. I've always felt that at some point, someone involved in the conspiracy would have sold out to the National Enquirer for $1 million." Is there anyone whose judgment he respects who believes the Mafia did it? "My wife, Alexandra, does," he tells Avenue. Schlesinger opines that Bill Clinton should not have been impeached because, in his own mind, Clinton was not committing perjury when he denied having sexual relations with Monica Lewinsky because the ex-prez is one of the Southern-school types who believes that oral sex doesn't count.</p>