Nate Silver reviews Gallup's likely voter model:

According to one formula that models turnover in the House based on the Gallup likely voter model specifically, a 13-point lead for the G.O.P. would translate into a gain of 71 (!) seats and an 18-point, lead, a gain of 86 (!!) seats.

Our version of a House forecasting model, which uses both Gallup and other generic ballot polls, as well as a whole host of other inputs, suggests that such gains by the Republicans are theoretically possible but rather unlikely. As of our last forecasting run, the model had given Republicans about a 1-in-15 chance of a gain of as large as 71 seats, and a 1-in-200 chance of a gain of 86 seats or more.