That gap has hovered in the 6 to 10-point range in recent weeks.
But Democratic voters tend to live in more concentrated areas and the party would typically needs a more significant advantage in the generic ballot number than do Republicans, whose voters are spread through more exurban and rural districts.
>Democrats need a higher share of the midterm vote to win a majority of seats, in other words, than do Republicans.
In 2014, when Democrats led the generic ballot in both CNN and NBC/WSJ surveys by only slim margins, Republicans ended up winning 13 Democratic-held seats.
>The last time Democrats gained significantly, in 2006, the party held a 15-point advantage in both surveys.
By contrast, Republicans enjoyed a generic ballot advantage of between two and six points in late October 2010, according to CNN and NBC/Wall Street Journal polls, but that narrow edge still resulted in the 63-seat gain..
Exactly party distribution matters... Dems have huge depth in very limited geography, while GOP enjoys good depth over a very broad area.. a true 50-50 vote split mathionally, means the GOP holds the house and by a solid majority.
I am not a huge fan of the generic ballot polling, as it ignores completely what is going on in the individual districts... but historically you can draw some general conclusions.