"Among the more prolific polling firms, the most accurate by this measure was TIPP, which conducted a national tracking poll for Investors’ Business Daily. Relative to other national polls, their results seemed to be Democratic-leaning at the time they were published. However, it turned out that most polling firms underestimated Mr. Obama’s performance, so those that had what had seemed to be Democratic-leaning results were often closest to the final outcome.
Conversely, polls that were Republican-leaning relative to the consensus did especially poorly."
http://fivethirtyeight.blogs.nytimes.com/2012/11/10/which-polls-fared-best-and-worst-in-the-2012-presidential-race/#more-37396
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