The Republicans went on to South Carolina. The next day Bush trounced McCain. On to Michigan. Again, the polls were wrong. After each fiasco, journalists explain that polls are only a snapshot of the moment.
People change their minds.
Or decide at the last minute. But I'm not so concerned about the accuracy of polls. I'm concerned about journalists using polls to frame their stories. Journalists covering the presidential election check four tracking polls every day. This gives them their fix of who's up and who's down since the previous day.