The narrowing of the Democratic lead raises questions about whether the party will win a large enough share of the popular vote to recapture control of the House of Representatives. The relationship between a party's share of the popular vote and the number of seats it wins is less certain than it once was, in large part because of the increasing prevalence of safe seat redistricting. As a result, forecasting seat gains from national surveys has become more difficult.

The Republicans also have made major gains, in a relatively short time period, among independent voters. Since early this year, the Democratic advantage in the generic House ballot has been built largely on a solid lead among independents.

Moreover, Republicans have gained ground in recent weeks on measures aimed at assessing a voter's likelihood of voting. So while Pew polls in early October and mid-October showed virtually no change in the Democratic advantage between all voters and those most likely to turn out, the current survey shows the Democrats' eight-point lead among all registered voters narrowing considerably among likely voters. In this regard, the current campaign more closely resembles previous midterm elections since 1994, when Republicans also fared better among likely voters than among all registered voters.