Reading Nate Silver’s 538 blog I see commentators wondering whether the margin in the polls naturally tightens as the election approaches. For 2008 I can say with some certainty that the answer then was no. For the current election, the results are more uncertain.
The nitty-gritty details are posted in the Technical Appendix. The headline results appear as those two solid lines that depict how the the competitiveness of each race has changed as they get nearer to Election Day. For 2008 we have all the polls over the last 120 days of the campaign, while for 2012 only polls conducted between July 7th and and October 2nd are included.
I use the absolute value of the difference in support for the two candidates as the measure of competitiveness. Races where Obama leads by two and races where Romney leads by two are treated identically. I have also restricted the analysis to the 120 days prior to the election (beginning in early July) after the nominees have been decided but before the conventions.
Despite this evenhanded approach, there is not much difference between “competitiveness” and how well Barack Obama is doing. Both John McCain and, so far, Mitt Romney have only held the lead on 12% of their polling days within the last 120.
You can see this at work in the closing stages of the 2008 campaign where the red line depicts Obama’s march to victory. There is no evidence of the 2008 race drawing closer as the election came near.
In 2012, we see a much different story, one of a campaign stuck in neutral for weeks now. The model predicts that the margin between the 2012 candidates will still be around 3.5% on Election Day as it has been for the past three months.
I’m going to add the 2000 and 2004 elections to this analyais. That will add another open-seat contest and one where an incumbent faces reelection. We might guess that 2000 will look more like 2008, and 2004 more like 2012. We’ll see how well this hypothesis fares in a later installment.