" Kennedy, 38, his wife and her sister are presumed dead after the plane he was piloting disappeared Friday night into the Atlantic off Martha's Vineyard. Because of "the Kennedy tragedy," Clinton postponed a three-day upstate swing she was to begin in Elmira today as part of her "listening tour" of New York, spokesman Howard Wolfson said. The trip will be rescheduled within the month. In recent years, JFK Jr. had been on the "dream team" of top state Democrats, one of a few marquee names party leaders hoped to recruit for a statewide race in New York. Public Advocate Mark Green said the results of a private poll he reviewed in 1997 before he ran for Senate confirmed why Democrats were hot for Kennedy. "He was by far the most popular Democrat in the state," Green said, declining to reveal who commissioned the survey. "He had the highest favorable-unfavorable spread; 65% of Democrats rated him favorably and 10% rated him unfavorably.