Why Do Some Midterm Polls Look Better for Democrats?

Nate Silver says asking people whether they will vote, and how enthusiastic they are about voting, produces a very Democratic sample. Other pollsters try to predict whether people will vote from other kinds of data, and the techniques tend to produce more modest results for Democrats.

Early voting is one place to look, though he generally advises people not to rely on it. Voters who have voted in no more than one of the past three elections, counting midterms, appear quite Democratic so far. Republicans will probably still cast votes on Election Day and catch up to some extent. If Democrats' low-propensity voters turn out and Republicans do not, a polling error might underestimate Democrats.