At Crystal Ball, Alan Abramowitz says that the September preelection generic ballot test is a good predictor of seat change in the House.
Democrats will need a lead of at least five points on the generic ballot in early September of 2018 in order to gain the 24 seats that they need to take control of the House. Based on the results of recent national polls, that number appears well within reach. On average, based on calculations from FiveThirtyEight, Democrats hold an adjusted lead of close to seven points on the generic ballot, mirroring that of the RealClearPolitics average. A lead of that magnitude would result in a predicted Democratic gain of close to 30 House seats, more than enough to regain control of the chamber. Given the model’s standard error of 11.6 seats, that forecast would give Democrats about a two-thirds chance of regaining control of the House.
So keep an eye on the generic ballot polling for 2018. If Democrats maintain a lead in the high single digits, that probably indicates they will have a decent chance to win the House or at least significantly cut into the Republicans’ majority. A bigger Democratic lead, into the double digits, would make a takeover more likely, while a smaller Democratic lead — or a GOP advantage — would put Republicans in a clearer position to preserve their majority.
