As it turns out, though, this question — the “generic ballot” question — is:

A bad poll question, A bad predictor of control of the House more than a year out, and A bad predictor of the size of the majority in the House immediately before the election.

RealClearPolitics compiles an average of generic-ballot polling over the course of a campaign. We can use that data to prove points 2 and 3 above, which, once proven, prove point 1.

Let’s compare, for example, the daily margin between the Democrats and Republicans on the generic ballot with the final margin between the parties in voting. At the end of the past four elections — two presidential and two midterm — the polling average has deviated between 1.4 points and 3.5 points from the eventual margin between the parties. That, by itself, is not actually bad.

Right now, though, we’re about 400 days from the 2018 election. This far out from the election in 2010, 2012 and 2014, the generic ballot average was 9.2, 0.4 and 7.9 percentage points away from the eventual result. That second one, from 2012, is pretty good. The other two are not, overestimating Democratic vote totals by 8 or 9 percentage points.

But the House isn’t determined by the results of a national poll. It’s determined by 435 individual elections.

Because of how those races run and how the districts are established, the margin of seats won is generally much wider than the margin of votes won by a party. On average since 1982, the party that won more votes saw a spread of seats won that exceeded the vote margin by 7 points on average. In two cycles, 1996 and 2012, Republicans won more seats despite losing the overall popular vote between the parties.

What that means, then, is that polling that gets the national vote margin right is still going to be way off on the national seat margin. RealClearPolitics’ data backs that up.

In 2010, the final polling average was only 2 percentage points away from the seat margin in the House. (Republicans led the generic ballot by 9.4 points and held 11.3 percentage points more seats in the House.) The final polling average in each of the three most recent elections, though, overestimated the margin of seats held in the Democrats’ favor by 7 to 11 percentage points.

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And that’s the final average. Four hundred days out, as we are now, the polling average overestimated the margin in favor of the Democrats by between 8 and 15 percentage points.

In other words, if that NBC/Journal poll is as far off as the averages were in 2010, 2012 or 2014, the Republicans will end up with a 2 to 9.6-percentage-point margin in the House.

Why does the generic ballot consistently seem to overstate the success of Democrats? Post polling analyst Emily Guskin explained the gap by noting that “while more Americans identify as Democrats than Republicans, Democrats have particularly low turnout in midterm elections.” That’s one reason that the 2010 and 2014 estimates 400 days out in that last graph are further from the final tally than the 2012 figure.

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None of which is to say that the Democrats can’t or won’t retake the House (though it’s an uphill climb for a number of reasons). It’s simply to note that one should take the generic ballot question with a grain of salt.

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Or — particularly this far in advance of the election — just ignore it entirely for predictive purposes.

Update: I realized that RCP also has data for 2006 and 2008.

In 2008, like 2012, the 400-days-out average margin was fairly close to the final vote. It’s worth noting that, in each of these elections, the Democrats ended up winning the national House vote, by a wide and a narrow margin, respectively. The final averages were close to the final national vote, consistent with the data above.



The margin of seats held was actually fairly close in 2006 — only 2.5 points off. The final average in 2008 was actually more Republican than the seats held.