McCain Bump Prediction

Based on the analysis presented here, my prediction for John McCain's convention bump is 1.4 percentage points. Based on the way I measure bumps, McCain's share of the two-party vote in trial-heat polls conducted during the week after the convention should be just 1.4 points higher than it is now. The primary reason for this small prediction is that McCain is currently running ahead of where he "should" be and, hence, is unlikely to gain much more ground.

Right now, there is no sign of a convention bump for Obama, though it is still too early to tell. I'll provide a recap of both candidates' convention bumps in a couple of weeks.

Nominating Conventions--Who's Watching?

This post is brought to you by Joe Cera, a graduate student in political science at UWM.

One of the central functions conventions serve is as important group identity rallying points. They are party-themed affairs, and they accentuate partisan concerns and partisan thinking. And potential voters expose themselves to information disseminated during the conventions based on their pre-existing partisan affiliations. In other words, to a certain extent conventions are “preaching to the choir.”

Consider the pattern of voluntary exposure to the 2004 convention speeches of Bush and Kerry amongst different partisan groups:

These viewing patterns show who the convention audience is—largely the convening party's supporters and political independents. And this viewing audience helps explain the messages we'll be hearing in the next two weeks. So if you hear the candidates or their surrogates talking about reaching out to the other party, you know that the other party is generally not watching.

Obama Bump Prediction

As promised last week, I have a prediction for Obama's bump in the polls from the upcoming Democratic convention.

Based on his current standing in the polls (4.75 points behind his expected--based on my super-duper forecasting model--vote share), the fact that the convention starts seven days before the Republican convention, and accounting for slight decline in convention bumps over time, the convention bump model predicts a 5.5 percentage point bump in Obama's share of the two-party vote in trial-heat polls. This is slightly smaller than the historical average (5.9 percentage points), in part because the two conventions are closer together than any others during the period analyzed (1964-2004).

The prediction is based on the results of a regression analysis (below) of convention bumps, utilizing the variables described above, as well as a dummy variable for the 1968 and 1972 Democratic conventions (I toyed with labeling this the "lousy convention" control variable).


The model makes substantive sense and fits the data fairly well. Of course, there is still a fair degree of unexplained variation in bumps, as illustrated in the graph below. The smallest error was .45 points (Democrats, 1996), the largest was 5.49 points (Democrats, 1984), and the average absolute error was 2.45 points.


What all of this says is that there is a regular pattern to convention bumps and based on that pattern Obama should get a bump of around 5.5 percentage points. However, it is also clear that it is not at all unusual for candidate to over- or under-perform by a few percentage points.

Of course, the fact that McCain's convention begins just a few days after Obama's ends could have an effect on the magnitude of the post-convention bump. While the model controls for the number of days between the beginning of the conventions, there are no cases in the data set where the conventions are this close together. In other words I am predicting outside the range of the data, always a dicey enterprise.

I should have a McCain Bump prediction about this time next week.

V.P. Pick

...no data

...no model

...just a guess

Hillary Clinton

Why? Well, all the party unity stuff, of course. But also, since she's generally not on many of the beltway lists, picking her would be a bold and unexpected move. And despite her long tenure as First Lady and Senator, the ticket would "look" more like a "change" ticket than one with Biden/Kane/Bayh at the number two spot. Of course, Obama/Sebelius would accomplish that as well (no, I'm not hedging--unless of course he picks Sebelius).

I'm very familiar with all the reasons for not picking Clinton, but I think there's a huge upside as well.

I'll also go out on a limb and predict that the pick will be announced just prior to prime time news broadcasts tomorrow (Wednesday). Okay, maybe Thursday (hedge). Waiting any longer would be silly.

Feel free to leave your pick in the comments section.

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.

Post-Kaiser-Harvard Poll

The Washington Post is reporting the results of the Post-Kaiser-Harvard Survey of Low-Wage Workers, which shows that working class voters--even white working class voters--are tilting decidedly in favor of Obama over McCain. Besides the fact that the Post is reporting poll results that are almost a month old, the other thing that struck me as odd about this survey is that it is probably a good example of question-order contamination and economic priming. The first thirty-five questions focused on many different aspects of personal and national economic attitudes, and then the thirty-sixth question asked respondents how they would vote if the election were held today. It's hard to imagine that the emphasis on economic questions throughout the survey did not have an impact on how respondents answered the vote question.

Imagine if the vote question had been preceded by thirty-five questions that focused on terrorism and foreign policy. My guess (I have no data) is that responses to the vote question would have reflected greater weight given to foreign policy attitudes and Obama's margin would have been much narrower.

The upshot is that Obama supporters who were excited by these poll results might want to temper that excitement just a bit. Also, poll consumers should always read the questionnaire.

Update: I just saw that Mark Blumenthal has a post at Pollster.com making many of the same points.