Why some polls may be underestimating Democrats
(CNN)Republicans dominated the midterm elections in 2010 and 2014 in part because voters who supported Barack Obama in presidential years voted for Republicans or stayed home when he wasn't on the ballot.
The turnout advantage Republicans enjoyed during the Obama years may no longer exist as we head into 2018, and in fact, there are some signs that Democrats may be the one with a decisive turnout advantage this cycle.
This shift in reality may not be reflected in much of the high-quality polling so far this year because much of it has been done among registered voters instead of the more refined category of likely voters. In other words, current polling may not fully reflect Democratic strength heading into the midterms.
Monmouth University has taken into account the gap between registered and likely voters. Monmouth has polled seven House and Senate races so far. In each of them, Monmouth identified "likely voters" based mostly upon a high propensity for voting in the past. Additionally, those voters who had a high-level interest, even if they haven't historically voted in midterms past, are included.
This model I feel is one that captures both the fact that past voting is usually highly predictive of future voting and that there may be some new voters casting ballots this year.
In five of the seven races polled, the Democratic candidate either did better or no worse when Monmouth switched from registered voters to likely voters. And in the two races where the Republican candidate does better with the switch to likely voters, the change was 2 points or less.
https://www.cnn.com/2018/07/04/politics/why-some-polls-may-be-underestimating-democrats/index.html
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