As we head into the convention phase of the campaign, I thought it would be useful to review the extent to which the convening party candidate receives a bump in the polls during their convention. I've covered much of this in previous posts so I'll just briefly review a few key points here.
The figure below summarizes the size of convention bumps from 1964 to 2012. The convention bump is measured 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.
There is a lot going on here but there are a few takeaway points.
1. Candidates generally get a bump of some sort. The size of the bump is highly variable but virtually all candidates leave their convention doing better in the polls than when they went into the convention.
2. The size of the convention bump does not predict the overall winner very well. Just ask Presidents Goldwater, Mondale, Dole, or Gore, all of whom had bigger bumps than their competitors. One of the reasons for this is that candidates running way behind in the polls have an easier time gaining ground during their conventions. For instance, in 1964 Barry Goldwater was so far behind in the pre-convention polls (averaging 21% of the vote) that it was easy for him to improve his standing by thirteen points during the convention, though he still never come close to being competitive. At the same time, Lyndon Johnson went into his convention with 69% of the vote in pre-election polls and left the convention with no bump but still with a substantial lead.
3. Convention bumps aren't what they used to be (see figure below). Prior to the 2000 election, convention bumps averaged more than six points, but that has fallen to just over two points from 2000 to 2012. One potential explanation for this change lies in the scheduling of conventions. The 2008 and 2012 conventions were held in late August and early September and were also held on back-to-back weeks. The norm in other years had been to hold the conventions in late July or early August and to separate them by two to three weeks. What is probably most important here is holding the conventions on consecutive weeks, which means that the convention messages end up overlapping and may cancel out each other. Another potential explanation lies in the increased polarization of the electorate. It is possible that partisans are so much more committed to their candidates now than they were before and there is a much smaller persuadable electorate that can be influenced by events like the nominating conventions.
So what does all of this mean for the 2016 convention bumps? One of the the key features of the 2016 conventions is that they follow the recent scheduling trend of back-to-back convention weeks, so this might limit the size of the bumps. On the other hand, since the conventions are being held in late July rather than late August, there might be more persuadable voters than in 2000, 2008, or 2012. One other factor that could lead to more substantial convention bumps is that both candidates have problems within their own party and the conventions present them with an opportunity to rally the base in a way that no other campaign event can. The key for both candidates is to have a smoothly run convention that heals rather than exacerbates existing party wounds and projects a positive message to the rest of the country.
