He left office nearly three years ago as one of the most successful Presidents ever, and Bill Clinton, the youngest ex-president in modern times, is still the go-to guy in Democratic politics. All of the party's major candidates for president say they call on Mr. Clinton for advice. They say the former president always seems eager to talk politics, and one says the former president sometimes seems to relish the calls as an excuse not to work on his memoirs. "He said to me once, `I shouldn't be spending this much time, I've got to be writing my book,' "...