A few hours ago, Mitt Romney announced Wisconsin representative Paul Ryan as his running mate. It seems obvious that in picking Ryan, Romney is trying to solidify his position with fiscal conservatives and doubling down on the economy. Personally, I think it’s a smart choice (see Bernstein for his thoughts). But the important question is: Do vice presidential candidates have significant effects on presidential election outcomes? This question is one that those in the media will presume to know, fueling endless speculation about the purported “value” of Romney’s decision. Drink every time you hear phrases like “game changer,” “transformative,” “shakeup,” or “high-reward.” We’ll see you back here next week… But this issue is one political scientists have studied, and most works uncovers only minimal effects on a president's vote-share. Typically, researchers in this field have looked for evidence of a VP's “home state advantage.” Though this is just one way to examine the topic of a vice president's electoral value, it seems that if the home state effect is minimal, there is little reason to believe a vice presidential candidate has significant electoral value nationwide. The conventional wisdom—see here for a paper by Dudley and Rapoport— is that vice presidential candidates do increase a presidential candidates vote share, but only in less populous states like Wyoming (Dick Cheney) and Delaware (Joe Biden) where the likelihood of swaying the Electoral College is minimal. A more recent study by Devine and Kopko adds the qualification that this effect only exists when the state is less populous and the vice president has significant political experience in his or her state (which I’ll concede Paul Ryan has). Still, further analysis has shown that the magnitude of this effect is quite small. Research by Garand (1988) and Rosenstone (1983) shows that a president’s vote-share increased by between 2.5% and 3% in his VP’s home state (cited in Devine and Kopko). In case you’re wondering, Obama won Wisconsin 56% to McCain's 42%.
Thus, while the strategic selection of a vice president does seem tied to the electoral value of a candidate in the Electoral College (see a paper by Sigelman and Wahlbeck), political scientists have not found systematic evidence that a VP has significant effects on election outcomes. Now admittedly there are other, perhaps more subtle but nonetheless important, ways a vice president can impact a presidential race. Ryan may help Romney frame the political right’s economic message and keep the economy forefront in the minds of voters, for example. But the purported value of Romney’s choice should be nuanced and tempered; this is not what we’re going to hear in the media over the next week or so.