I'm proud to count myself among the chorus of adherents to the scientific method even in non-scientific settings. To stereotype, in baseball, these people are the sabermetricians; in politics, they're the political scientists. Yet despite their rising profile and their many successes, their counsel and predictions are often ignored in favor of that of scouts and campaign operatives. Again, while this is a stereotype, these are the people who are more apt to draw false conclusions based on a perceived pattern from their years of experience.
We're treated to that every four years on the grandest scale, when at least one major party nominee chooses a vice-presidential running mate. Conventional wisdom among the media, political professionals, and basically everyone else holds that it's the most important strategic decision a campaign can make—capable of delivering the votes of entire demographics and, ideally, a vice-presidential candidate's home state in the Electoral College.
There is no hard evidence for this. What's more, there is no soft evidence for this. In fact, it has been demonstrated time and time again that there is no "home-state advantage" for a vice-presidential nominee's ticket. Consequently, while Mitt Romney may have his own very good reasons for picking Senator Marco Rubio or Senator Rob Portman as his number two, he should be careful not to do it thinking it gains him an edge in their respective home states (Florida and Ohio, two of the biggest swing-state prizes out there). Unfortunately, Romney is getting advice to the exact opposite effect. Not only does the media speculate endlessly about home-state advantage when it comes to a VP pick, but Republican insiders place a premium on it too. One Portman proponent drew the connection as starkly as it could be drawn: "He's from Ohio, and we need to win Ohio."
This assumption is fostered by a lot of anecdotal, and some scientific, evidence. Just this week, a CBS News/New York Times poll found that 26% of respondents claimed the vice-presidential nominee mattered "a lot" to their vote, and 48% said it would affect their vote "somewhat." Unfortunately for campaign operatives picking a VP, however, this is a long way from proof that their selection matters. It is one thing for people to say in the abstract that the bottom of the ticket is important; it is another to present them with different possible tickets (e.g., Romney-Jindal, Romney-Pawlenty) and see if that truly sways their opinion.
Indeed, few and far between are the polls that show an actual, concrete vice-presidential pick making a difference outside the margin of error. For instance, Fox News polled Ohio residents about a generic Obama-Romney matchup, but then asked about a more specific Obama-Biden vs. Romney-Portman matchup. The change between the two amounted to little more than noise in the data; the control resulted in a 45%–39% Obama edge, while the inclusion of the VP candidates made it 46%–40%. That's a one-point change for each candidate, and a net change of zero.
Similarly, in a poll of Florida, Public Policy Polling discovered that putting Rubio on the ticket actually decreased Romney's share of the vote by two points—from 45% to 43%. Worse, President Obama's share of the vote remained steady at 50%. Another rumored vice-presidential candidate, Jeb Bush, also barely registered an impact. He shaved one point off Obama's vote share and gave one point to Romney, for a 49%–46% race. And this was the most benefit given by any possible VP nominee that PPP tested! Not only does this show that a VP nominee changes very few minds, but it also pokes a giant hole in the belief that, even if the nominee does change minds, it provides an automatic bump in the nominee's home state. Yet the belief in the importance of the bottom of the ticket persists.
Granted, there are polls that show the opposite result—that is, a vice-presidential candidate making a difference. A May poll by Quinnipiac, for instance, was upheld as evidence that Rubio would help Romney eke out a win in Florida. But even that poll showed only a two-point boost with Rubio, with the article noting below the (e-)fold that that difference was within the poll's +/-2.4% margin of error. The point is that, for every poll showing the VP nominee making a difference, there is one that shows no effect—making the impact of vice-presidential selection basically inconclusive. Furthermore, if you can find me a poll that attributes a five-point bump to any vice-presidential candidate (and if you can, please send it to me), please also find for me the last time a poll in July accurately pegged the final margin of victory in November. Circumstances change, and literally hundreds of political events will intervene between now and then. It is a reach to assume that people reacting in July to the naming of a vice-presidential candidate carry that reaction with them for four months and have it continue to weigh on their vote on Election Day.
The reality is that people think that their vote is based partially on the VP half of the ticket. They think of themselves as making a thoughtful, balanced choice, considering many variables (of which the ticket is one)—so they tell a pollster that, yeah, it's somewhat important to them. (This is also the reason that political operatives think it matters.) But when they make that choice, how often is the vice-presidential nominee the tiebreaker? Or, realistically, how many people like most things about the Democrat but then decide to throw all that out the window because the Republican vice-presidential nominee is from their state? How much sense does it make to base your vote on the number-two guy over all of the factors to consider—which are about the number-one guy?
Indeed, most people who really like the number-two guy enough to be swayed to vote for him (or her) already like the number-one guy, too. In other words, they are already strong partisans or votes that are already in the bag—even if they didn't seem to be. I concede, for example, that a wavering evangelical could be reassured into voting for Romney by a solidly conservative pick like Chris Christie. (Indeed, these were the findings of a PPP national poll in April: "Christie's inclusion doesn't have a big impact with Democrats or independents but it helps Romney shore up the party base, going from an 82-10 lead with Republicans to an 86/9 one.") However, as Election Day approached, those waverers would have coalesced around Romney anyway, in a well-documented political-science phenomenon known as "coming home" to the base.
This overall theory lines up pretty well with the empirical findings of political scientist David W. Romero, who conducted one of the most definitive studies on the issue of the vice-presidential nominee and voting patterns. Reviewing the political-science literature, Romero found—as we did—that surveys of the individual voter gave the vice-presidential candidate a lot of weight, but that that impact disappeared when looking on the aggregate level—removing from the equation individuals' "rationalization" of their vote. He divided up the factors that influence electoral decision-making into two camps—those pushed from the campaign to the voter (e.g., ads) and those intrinsic to the voter (e.g., partisanship). However, he found that most of the campaign-generated persuasion was geared toward the presidential candidate. The little that was related to the VP candidate tended to be grouped around the summer, which another study tells us is almost irrelevant to November (campaign ads during the summer just don't have the "half-life" to continue to influence people months later). As a result, the only way the vice-presidential nominee can influence a voter is by appealing to his or her intrinsic factors; or, as Romero hypothesizes, "the voters’ evaluation of the vice presidential nominees is at best flimsy and impressionistic, and at worst simply a function of their predispositions." When he controlled for those predispositions in his study, he found that, no matter how much voters thought they were considering the VP nominee, it simply didn't switch any votes in practice.
If you don't trust Professor Romero, we need look no further than trusted election guru Nate Silver's simpler, more plain-English study concluding the same thing: that the vice-presidential nominee barely moves the needle in his or her home state. Silver took just about the most straightforward approach you can to this problem: he looked at how well tickets did in the home states of their VP nominees relative to their nationwide performance. His findings averaged out to a 2.2% bump for the hometown VP's ticket—not bad, but certainly not a game change. In addition, the historical range fluctuated everywhere between a 21.1% bump (John Nance Garner in 1932) and a 17.8% disadvantage (Joseph Taylor Robinson in 1928). In other words, the effect is wildly unpredictable.
Yet despite this mountain of evidence, there continues to be a massive disconnect between reality and perception when it comes to the selection of a vice-presidential nominee. That nominee almost never provides the requisite boost to put a state over the top for his or her ticket, and that state has almost never been the difference-maker in the Electoral College. Still, however, prognosticators today continue to talk about swing-state status as the hot-button issue in the veepstakes. Can they actually believe this to be true? With all the evidence coming down against it, and the almost complete lack of proof to support that belief in the first place, there is absolutely no reason for anyone to believe that a vice-presidential pick makes a difference in how people vote.
No, votes for the presidency tend to boil down to the very things you'd expect them to: partisanship, ideology, presidential approval rating, and, especially this year, the state of the economy. When it comes to Mitt Romney's choice of a running mate, the best advice for any presidential campaign continues to be, "Keep it simple, stupid!"