Convention Bumps

Many years ago, when I was writing Do Campaigns Matter?, I had a lot of fun working on the chapter on conventions. As the 2008 conventions draw near, I thought it would be useful to update some of the earlier analysis to get a sense of how big a boost the candidates can expect from their conventions.

The figure below presents the magnitude of the convention "bumps" from 1964 to 2004. The bumps are calculated using Campbell, Cherry, and Wink's (1992) measure: the candidate's average share of the two-party vote in trial-heat polls conducted six days to two weeks prior to the start of the convention is subtracted from his share of the two-party vote in polls conducted during the seven days following the close of the convention. The change in poll standing is the the convention bump. I'm sure there are a lot of plausible alternative means to capturing convention bumps but I assume they would all tell a very similar story. Anyway, here are the data (Republicans in red, Democrats in Blue):

(Click on image for a clearer picture)

The first thing to note is that there is a lot of variation in convention bumps. Fortunately, as I showed in Do Campaigns Matter?, there is a systematic component to that variation. Two things in particular seem to drive the size of the bumps. First, candidates who are running ahead of where they "should" be (based on the expected election outcome) tend to get smaller bumps, and those running behind their expected level of support get larger bumps. In this way, the conventions help bring the public closer to the expected outcome and help to make elections more predictable. The perfect example of this phenomenon is the 1964 conventions. Goldwater got a huge bump, in part because he was running 16 points behind his expected vote share, and Johnson got no bump, in part because he was running 6 points above his expected vote share. Likewise, Al Gore was running well behind his expected level of support in 2000 and got a substantial bump, while George W. Bush was running ahead of his expected level in the same year and received a rather modest bump.

The other key factor is the timing of the convention. The earliest convention tends to get a bigger bump, and there is some evidence that going appreciably earlier exacerbates this effect. One reason for this is that the first convention is held by the out-party, whose candidate generally less familiar to the general public. Another possible reason is that opinions are generally less well-formed early in the campaign and may be more easily shaped by the first convention. In the data presented above, the first convention bump was the larger than the second in seven election cycles, smaller in three (1988, 200, 2004), and essentially tied in one (1980).

A couple other considerations. It should be clear that the magnitude of the convention bump is not a great predictor of election outcomes. For instance, Goldwater got a huge bump and went on to a miserable defeat, Nixon got a huge bump and narrowly won in 1968, and Reagan got a very small bump and still won the 1984 election by a wide margin. My sense is that the size of the bump doesn't matter so much as whether the candidate gets the bump he is expected to get.

Finally, each of these conventions has it's own story, and it is easy to come up with plausible explanations for why candidates did better or worse than might have been expected. For instance, the 1968 and 1972 Democratic conventions were disasters, and this is reflected in the convention bumps (or dip, in 1972). Also, John Kerry's minuscule bump in 2004 might be attributed to the fact that he announced his vice-presidential choice in early July rather than during the week prior to the convention (which is what most candidates do), or to the fact that the Swift Boat ads were on the air within days of the end of the convention. At the same time, we also have to consider that Kerry was leading Bush in the polls prior to the convention, so he was unlikely to increase his poll standing very much.

Even with these sort of stories, there is a pattern to convention bumps that should be a useful guide to what to expect later this month.

In about one week, I'll have the final pre-convention polling numbers for the Democratic convention. At that time I'll provide a final, precise prediction for the Obama's bump, along with the regression model (which incorporates the data presented above) used to produce the prediction. In the meantime, given where the candidates stand today, assuming the general pattern in the data holds up, all indicators suggest a substantial (though not huge) bump for Obama and a somewhat smaller bump for McCain.