[T]his kind of overclaiming isn’t limited to media pundits. Campaign operatives do the exact same thing, including even skilled political strategists who follow the polls closely and probably know very well what their candidates’ true chances are. The real answer to Smith’s question probably has to do with the “bandwagon effect.” A small but significant number of swing voters tend to support whichever side seems to be winning, partly because they want to be identified with a winner and partly because of a sense that whoever seems to be winning might well be the best person for the job for that very reason.
Will Wilkinson weighs in:
It is illuminating to see journalists and pundits as participants in a subtle, subtextual status game, in which they vie to improve their guy's status relative to his hated opponent. Loudly touting polls that show Barack Obama struggling among working- and middle-class whites is a way of communicating that support for Mr Obama may be an imprudent status move among America's largest demographic group. Counter-stories suggesting that Mr Obama's trouble with whites is a sign of lingering racism can be a way of communicating to America's largest demographic group that, by refusing to support the president, one risks raising suspicions of America's original and, at this point in history, trashiest vice.