A favorite topic of Eddy Elfenbein. (I gave him an award for that in 2005.)

As of the last trade, the market believes the Democrats have a 60.2% chance of gaining at least 19.5 seats, and a 46.9% of gaining 24.5 seats. A 60.2% chance is +0.258 standard deviations, and a 46.9% chance is -0.078 standard deviations. So those 5 seats are worth 0.336 standard deviation. Therefore, one standard deviation is nearly 15 seats. The Democrats are now projected to gain over 23 seats, but the market still believes its wide open.