Racists in America must be having a field day: At long last, they have found a world-renowned intellectual -- Harvard's Academy for International and Area Studies Chairman Samuel Huntington -- to rationalize their resentment against America's rapidly growing Hispanic community. Huntington, whose 1993 book The Clash of Civilizations was later credited for having foreseen the Sept. 11, 2001, terrorist attacks, says in his forthcoming book Who We Are (Simon & Schuster) that the United States is threatened with national disintegration because of the soaring rate of Hispanic immigrants. ''The single most immediate and most serious challenge to America's traditional identity...