|"Dewey Defeats Truman", a famously inaccurate headline, Chicago Tribune,1948, from archives.gov|
I found these 2 prediction markets for the US elections: Intrade presidential and senate race, and IEM presidential race.
|Intrade prediction market: Obama vs. Romney|
|IEM prediction market: Obama vs. Romney|
Oracles: How Prediction Markets Turn Employees into Visionaries by Don Thompson (Chapter 4, p. 47), IEM prediction market was more accurate in predicting elections than than the polls 74% of the time, based on a total of 964 polls, conducted 1988-2004. This seems surprisingly more accurate. As they say, "prediction is very difficult, especially about the future".