It's hard to say what we should expect in terms of polling gains from the COVID-era, remote nominating conventions over the next few weeks. On one hand, the well-choreographed convention floor "show" will be missing, potentially robbing the both Biden and Trump of whatever benefits there are from the spectacle nature of these sorts of events. At the same time, the candidates are still the beneficiaries of four straight days during which they probably will dominate media coverage, including two-hours of prime-time television coverage each night of the convention. So it may well be that the impact of the conventions in 2020 are not terribly different than in other years. We'll know more about this in a few weeks.
In the meantime, I thought it would be useful to review the extent to which the convening party candidate receives a bump in the polls following 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 2016 (Red=Republican, Blue=Democrat). 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, and different calculation methods (dates covered, polls used) could produce different results.
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 2016. 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 from 2008 to 2016 the conventions were 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 2020 convention bumps? One of the the key features of the 2020 conventions (besides the obvious, COVID-inspired changes) is that they follow the recent scheduling trend of back-to-back convention weeks, so this might limit the size of the bumps. In addition, since both conventions are being held in late August, the pool of persuadable voter who have not already made up their minds could be fairly small. Finally, one thing that might limit Biden's bump potential (and hint at a larger bump for President Trump), is that he already has a substantial lead in the polls and there might not be much room for his lead to grow. If Biden does manage to get a substantial bump, it may mean that his vote potential is higher than his current polling level.
