The other key point is this: Late movement in Senate polls tends to be in the direction of the underlying fundamentals. I discussed this movement in the polls in an earlier post, and it’s worth revisiting it now.
The analysis is pretty straightforward. Estimate a simple model of Senate elections from 1980 to 2012 that relies on only a few factors: economic growth, presidential approval, whether it’s a midterm or presidential year, and how the state voted in the most recent presidential election. Then estimate an out-of-sample forecast for every Senate election between 1992 and 2012. Then compare the polls to that forecast.
Here is the gap between the polls and the forecast for the last 60 days of the campaign:
On average, the polls have not yet fully incorporated even these basic fundamentals by this point. After today, 18 days before the election, there tends to be further movement toward those fundamentals. The sharp movement in the last few days of the campaign should be taken with a grain of salt, since the volume of polling drops over the weekend before Election Day, but the overall pattern is clear.
As we noted back in January, a slightly more elaborate fundamentals-based forecast based on elections since 1980 favors the Republicans. And this is what poses a challenge for the Democrats: Right now they need the polls to move in their favor, but in key states this would entail movement opposite to what the fundamentals would predict and therefore opposite to the trend above.
Of course, it’s not impossible for polls to move in that direction. The graph depicts an average across many Senate races, and not every race may conform to this pattern. It’s just unlikely that the polls will move enough in that direction for Democrats to benefit on Election Day.