The results are something of a mixed bag, but overall it isn’t clear that the pollsters have really fixed the problem at all. While the bias toward Democrats was smaller in 2018 than in 2016, the bias overall was similar to what we saw in 2014, especially in the Midwest. If people remember, the polls in 2018 suggested that we should today have Democratic governors in Ohio, Iowa and Florida, and new Democratic senators in Indiana, Missouri and Florida. Obviously this did not come to pass.
Moreover, almost all of the errors pointed the same way: Republicans overperformed the polls in every Midwestern state except for Minnesota Senate/governor and Wisconsin Senate (none of which were particularly competitive). This is true, incidentally, across the time period: We see marginal Democratic overperformances in the Michigan and Minnesota Senate races in 2014, but otherwise pollsters have consistently underestimated Republican strength. Note that if we had added the competitive Senate race in North Dakota and the governor race in South Dakota in 2018, we’d also see Republican overperformances of a couple of points.
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