I measure convention bumps as the percentage point change in the convening party's share of the two-party vote, comparing polls taken between six days and two-weeks prior to the convention with polls taken during the seven days following the convention. Note that this is a short-term measure of the convention bump and does not say anything about the rate of decay in the weeks following the convention. The figure posted below summarizes the convention bumps for both the Republican and Democratic nominees from 1964 to 2008.
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. A 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 prior to the Democratic convention. For the years presented above, the correlation between how far ahead of the expected outcome (based on an election forecast) a candidate is running in pre-convention polls and the size of the convention bump is -.42. While all candidates may want to get a big bump from their convention, big bumps are not always a good thing; they could signal that the campaign is not doing as well as expected.
Timing also plays a role in explaining convention bumps. When I first started looking into this (1996) there was a tendency for the party holding the earliest convention to get a larger bump. This made sense for a couple of reasons. First, it is the out-party holding the earliest convention and people usually know less about the out-party nominee, since the incumbent party almost always runs an incumbent president or vice-president (John McCain being the most recent exception). In addition, historically the first convention had been held sometime in late July or early August, a time period when there might be more undecided voters. Importantly, the tradition at the time was that the in-party convention would usually be held three to four weeks later, which gave time for the message from the first convention to resonate. However, in 1996 and 2000 the conventions were held fairly late and only two weeks apart. And, more recently, the 2008 conventions were held in back-to-back weeks in late August and early September, creating a situation in which the messages of both campaigns no doubt interfered with each other. This, of course, is also the timing of the 2012 conventions. When taking these changes in timing into account, it turns out that the number of days a convention is held before or after1 the other party's convention is a stronger predictor of convention bump (r=.37) than simply going first (r=.22). Of course, the "days between conventions" measure captures both how early the first party convenes and how close together the conventions are.
Finally, it also appears that there is a general trend toward smaller bumps in the last few elections cycles, though this could simply reflect the changes in timing of the conventions. Truly large bumps were somewhat common prior to 1996 but have not made an appearance since then.
Taking all of these factors into account, and adding a dummy variable to control for the disastrous Democratic conventions of 1968 and 1972, we can see that there is a predictable element to convention bumps:
This is not to say that there aren't interesting and unique aspects of every convention that might explain why the convening party over- or under-performs the expected outcome--just that, by and large, the pattern of convention bumps is explained by a few simple variables. The correspondence between predicted (by the model shown above) and actual convention bumps illustrates this point quite well:
Generally, conventions expected to produce large bumps tend to do so, and those predicted to produce small bumps also fit the pattern. What's important to note, though, is the the pattern is far from perfect and some of the larger deviations from the from the predicted outcome make sense, given what we know about those specific conventions. For instance, the Democratic convention of 1984--a convention plagued by in-party fighting--underperformed by a little over five percentage points, while the Democratic convention of 1992--one that benefit from Ross Perot dropping out of the race and near-endorsing the party's nominee, Bill Clinton--exceeded expectations by almost six percentage points.
2012 Bumps
So what does this all mean for the conventions coming up in the next two weeks? I won't have complete data for the Obama bump prediction until the opening days of the Republican convention, and my general election forecast (used in the model) is preliminary at this point (I will post that model when all the data are in; preliminary data point to narrow Romney win). But I don't expect any of the data to change dramatically in the next few days, so I will go ahead and make a prediction for the 2012 convention bumps:
Predicted Romney Bump: 3.6 percentage points
Predicted Obama Bump: 1.1 percentage points
Right now, the two candidates are in a tight race, with Obama holding a slight advantage in the polls. Based on my bump predictions, I expect that the race will continue to be tight after the conventions but that Romney will hold a slight lead.
Huge caveat: Hurricane Isaac. I have no earthly idea how a hurricane-shortened convention will afect things, though I suspect it would not be good for the Romney campaign
1So, the first convention gets a score equal to the number of days it begins before the first day of the second convention, and the second convention gets a score equal to the number of -1 times the number of days it begins after the last day of the first convention.
