Zooming around the web is a suggestion that Presidential debates do not matter in determining election outcomes. A 2008 Gallup study claims “no direct correlation between the winner of each debate and the winner of the presidency.” Let’s ask two questions. First, is post-debate change measurable with a sensitive enough instrument? Second, is the change limited to the Presidential race, or can it spread downticket? I believe that Republicans and Democrats still have much to gain or lose from tomorrow’s debate. (Update: I see David Gergen and I agree.)
A figure from a new book by Erikson and Wlezien, The Timeline of Presidential Elections, appears to make the same point as Gallup:
As great as this graph is (I am finding their book to be quite a valuable resource), the claim above misses the mark in two ways.
First, let us test whether the debates have any effect on opinion using the 2004 and 2008 campaigns, cases in which the Meta-Analysis was available to provide precise information. As most PEC readers know, the Meta-Analysis is a more sensitive readout of national opinion than other poll aggregators. The bottom line: the first debate can drive opinion by the equivalent of several percentage points of popular vote margin.
The more clear-cut case is 2004. This was a remarkably close-fought race between John Kerry and President George W. Bush. The lead switched several times during the general campaign season. In the home stretch, Bush pulled it out – barely. You can see the history here:
It is fairly clear here that Debate #1 was followed by a jump in Kerry’s fortunes of almost 30 EV. Indeed, at the time Kerry was generally thought to have bested Bush. The next two debates had little further effect – though the initial jump did persist to the finish.
From a cognitive standpoint, it makes sense that the first debate would make a difference. Generally speaking, contrasts between two objects or persons are easier when they are side by side. The usual format of campaigns allows access to only one candidate at a time, making them hard to compare directly. For example, who is quicker on his feet? Heck, who is taller?
In 2008, the first McCain-Obama debate was followed by a giant move towards Obama:
Here there is a complication: just before the first debate, Lehman Brothers collapsed, setting off an economic meltdown. This appears to have sent the race spiraling toward the Democrats. Whatever the cause, by Gallup’s definition, it is not obvious that events from mid-September onward mattered for the Presidential outcome, since Obama was already leading.
If you compare this with the “08″ data point in the Erikson/Wlezien graph, you will see that the effect is large, but they concealed this by plotting total vote share, as opposed to before/after change. Their data did not allow a finer-grained measurement.
You can get a feeling for how large these effects are by examining the Popular Vote Meta-Margin. The approximate conversion is 15-20 EV/% Meta-Margin. A clear victory in tonight’s debate might swing opinion by up to 1-2%.
In this light, my modification to Gallup/Erikson/Wlezien is that the effects of the debates are usually smaller than the margin between the candidates, and therefore unlikely to flip an outcome. This year, even if Romney does extremely well, he will still probably make up only 1-2% of the >5% margin between him and President Obama. That is not enough. This is comparable to other events that Andrew Ferguson and I have marked on these charts: the Bain attacks (25 EV), the Ryan and Palin VP nominations (20-35 EV), and the conventions (0-30 EV).
However, all of this misses a point that may be quite critical this year. The Presidential outcome is only one measure of the debates’ effectiveness. As I have shown, downticket Senate races this year have tracked Presidential preference closely. The same is likely to be true of House races. These linkages are driven in part by partisan voter intensity. If one side’s intensity fades or surges, it will affect races at all levels on the ticket. The Obama-Romney debate may well influence the shape of the Congress that the President will face in January.