For political touts, no question in the presidential campaign deserves to be more hotly debated than the meaning, if any, of these national polls. From cable TV to newspaper front pages to poll-propelled Web sites like Pollster.com and RealClearPolitics, it is nearly impossible to resist the siren song of these dancing digits. They are so tangible, so seductively precise that, like SAT scores purporting to forecast college performance, they take on a phantom aura of certainty. This year, Hillary Clinton's wide lead has only increased the long-standing temptation to believe in the polls' predictive power.
But a strong case can be made that these polls are not as definitive as they seem -- that they are little more than the political version of dream books that use nighttime visions to predict winning lottery numbers. Mark Mellman, John Kerry's pollster in 2004 but nonaligned in this year's Democratic race, is a leading skeptic of national polls, stressing that they are invariably upended if anyone but the favorite wins the Iowa caucuses and the New Hampshire primary. In a column for the Hill earlier this year, he noted that "on average Kerry picked up about 20 points nationally from his Iowa win and another 13 from New Hampshire." Or as he put in epigrammatic fashion in an interview, "If there was a law requiring relevance, national primary polls would be illegal."
As a forecasting device, at least for Democrats, national polls have proved to be less reliable than a divining rod. Gamblers, in fact, would have done very well by following a surefire rule: Bet against the national polls in Democratic presidential races.
Since 1975, only twice has the candidate atop the Democratic field in the national Gallup Polls at this point in the campaign cycle gone on to win the nomination. The exceptions were Al Gore in 1999 and Walter Mondale (47 percent in the November 1983 Gallup Poll), who almost lost the nomination to Gary Hart, who was literally an asterisk in the same survey. And in the November 1991 Gallup Poll, a small-state governor named Bill Clinton was running sixth (yes, sixth) in the Democratic horse race, behind Mario Cuomo and such implausible presidential choices as Jerry Brown and Doug Wilder.
http://www.salon.com/news/feature/2007/10/26/polls/