Well - the 68% is not really a percentage: it's the price of a contract which will pay 100 to the owner if Obama wins. It's the market's consensus of what the probability is, so yeah it's a percentage. Given that the state-by-state percentages are also contracts, it seems that somebody with a little bit of time on their hands, and some programming smarts, could make a little money doing some arbitrage between the state contracts and the overall contract.