I don't have time to go into why public opinions polls are so problematic at election time.
But, there's a new predictive model out there, that's worth some attention. Fivethirtyeight.com is the brain child of Nate Silver. He takes a large swatch of polls, their current results of the "if the election were held to today, whom would you vote for" question, then plugs it into a statistical model that is then re-run dozens of times to establish the likely outcome of the election. His poll was an excellent predictor of actual outcomes during the primary.