Intrade.com runs a real-money security market for the democratic, republican and other candidates for each state. For example, If the democratic candidate wins the electoral college in Ohio the security OHIO.DEM security will pay off $100 otherwise it will payoff nothing. Traders can buy, sell, even sell short such securities. The price reflects a consensus value of the probability that the event will occur. If the price of OHIO.DEM is 48.3 that reflects a 48.3% chance that the democratic candidate will win Ohio.
When you load the map, the last sale prices from Intrade are converted into colors. The state is colored Red (Republican) or Blue (Democrat) depending on the maximum price with the shade indicating the amount of the lead. Near white represents a close race.
Put your mouse over a state to see the probabilities based on the normalized security prices for that state. Clicking on that state takes you to the Intrade site for the security of the leading candidate, including a graph of historical trading information.
Below the maps you can find the expected number of delegates each candidate will win as well as the delegate count if each state colored blue went for the democratic candidate and red for the republican. Finally we give probabilities for each candidate winning the election based on the normalization of two other securities PRESIDENTIAL.DEM2008 and PRESIDENTIAL.REP2008. To personalize the parties, we use the names of the presumptive nominees instead of the parties.
The map doesn't show the (heavily democratic) District of Columbia, but the DC securities are taken into account for the expected and the leaning counts. We also ignore the FIELD securities but may incorporate them later if a strong third party candidate emerges.
This map was built by Lance Fortnow based on similar maps for the 2006 Senate and Governors races designed by Fortnow, David Pennock and Yiling Chen. We decided to build those maps based on Electoral-Vote.com that uses poll data to color their map. We built this map in response since we believe that market prices give a truer reflection of how each state will vote. Market prices did much better than poll results in most recent elections Senate and Presidential elections. Also our prices update immediately as the markets react to new information.
Fortnow will plan to maintain these pages through the election but only have limited time to add other features on the page or the widget. He's happy to share the code if there is a programmer who would like to help add features or improve the interface.
For more information about the predictive power of prediction markets see Artificialmarkets.com and Chris Masse's extensive collection of prediction market resources.