Joe Biden grabs his mask after introducing his running mate, Sen. Kamala D. Harris (D-Calif.), during their first news conference together Aug. 12 in Wilmington, Del. (Olivier Douliery/AFP/Getty Images)

On Tuesday, presumptive Democratic presidential nominee Joe Biden named Sen. Kamala D. Harris (Calif.) as his running mate. If the Democratic ticket wins in November, Harris would become the first woman — and the first person of color — elected as vice president. Assuming a Biden White House follows recent precedent, Harris would serve as a top presidential adviser and troubleshooter at home and abroad. She would also be next in line for the presidency.

Does it matter who runs as the vice-presidential candidate? Our new book, “Do Running Mates Matter?: The Influence of Vice Presidential Candidates in Presidential Elections,” draws on decades of survey data to test three different ways that running mates might influence presidential voting.

Harris’s selection in and of itself is unlikely to change many votes, but her presence on the ticket is likely to influence voters’ opinions of Biden and whether they’re likely to vote for him.

Here’s how we did our research

In our book, we examine three ways in which the vice-presidential candidate might influence voters. First, if a running mate has a “direct effect,” her popularity influences which presidential candidate voters pick. Second, if a running mate has “targeted effects,” she influences vote choice among a specific subset of voters from a shared social group, which might be geographic, demographic, ideological or something else. Finally, “indirect effects” mean that a running mate influences voters’ decisions by shaping their perceptions of the presidential candidate.

To test for these effects, we used survey data from the 1952-2016 American National Election Studies. We also used other surveys that interviewed different sets of voters over the course of the presidential campaigns in 2000 and 2004, and still other surveys that interviewed the same sets of voters throughout the campaigns in 2008, 2012 and 2016. Our analysis relies on a variety of statistical methods, designed to explain why voters choose to vote for one candidate over another.

Here’s what we learned.

Voters don’t try to elect their favorite VP candidate

If you ask Americans during the campaign whether the choice of a running mate is important in deciding their vote, more than 80 percent say yes. But if you ask them after the election to explain in their own words why they voted for or against a presidential candidate, on average less than 5 percent mention the VP pick. And if you ask whether the choice of a running mate has ever changed their vote, only 10 percent of survey respondents say yes.

When we account for the other forces that drive presidential voting, the running mate’s popularity generally has a minimal effect on vote choice. In many elections, we see no effect at all. Even when the vice-presidential candidate becomes more popular during the campaign, the resulting effect on voters’ decisions usually fades away within a day or two.

In short, don’t expect Harris’s popularity, or lack thereof, to change many votes.

Running mates don’t typically “deliver” key voting blocs

Could Harris have a “targeted effect” by delivering a key group of voters — particularly women, Black or Indian American voters?

Picking Harris probably will not win Biden additional votes among women. We found that neither of the two previous female running mates — Democrat Geraldine Ferraro in 1984 or Republican Sarah Palin in 2008 — steered more female voters to their respective ticket. For example, our analysis of panel data from 2008, in which the same survey respondents were interviewed repeatedly over the course of the campaign, shows that women were no more likely to support the Republican ticket following Palin’s selection than they had been beforehand.

More recently, our analysis of panel data from 2016 shows that Mike Pence did not increase Donald Trump’s support among evangelical Christians, conservatives or Midwesterners. Nor did Tim Kaine increase Hillary Clinton’s support among Catholics or Southerners.

Of course, we can only speculate about Harris’s likely effect on Black voters, because no major party ever has nominated a Black vice-presidential candidate. But given the underwhelming evidence of targeted effects in past elections and Black voters’ already overwhelming support for Biden and the Democratic Party, we expect a modest effect, if any. Much the same could be said for Indian Americans, particularly given their already strong support for Democratic candidates.

Voters do judge the presidential candidate based on the running mate

Vice-presidential candidates matter mostly because of their indirect effects: They can change how voters see the presidential candidate who made the selection.

Harris’s political experience is likely to reassure persuadable voters about Biden’s judgment. In addition to having good credentials on paper — as a U.S. senator, former district attorney and former California attorney general — Harris already auditioned for the presidency during the recent Democratic primaries. As we show in our book, voters who perceive the running mate to be well-qualified also are more likely to think well of the presidential candidate’s judgment, and then to vote for the party ticket. Barack Obama’s selection of Biden as his running mate in 2008 had this effect. Biden’s selection of Harris probably will yield similar benefits in 2020.

Running mates also influence perceptions of the presidential candidate’s ideology. Harris, while presenting herself as ideologically progressive, is not clearly identified with the center or left wing of the Democratic Party. As a result, she may shift perceptions of Biden’s ideology a bit to the left, without alienating intraparty factions or swing voters.

Could this year be different?

Our research suggests that Harris could have a positive yet marginal effect on Biden’s chances in the 2020 election. But as we learned in 2016, the electoral past does not necessarily predict future results. As a Black woman, Harris probably will attract more media coverage and endure more aggressive attacks during the campaign than previous vice-presidential candidates. Indeed, the Trump campaign, which wants to portray Biden as susceptible to manipulation by left-wing Democrats, already is signaling that it will make Biden’s running mate a major issue in the campaign. In that case, while Harris’s candidacy probably will not decide the 2020 presidential race, she may matter more than most running mates.

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Christopher J. Devine (@ProfDevine) is an assistant professor of political science at the University of Dayton.

Kyle C. Kopko (@KyleKopko) is an adjunct professor of political science at Elizabethtown College.

They are the co-authors of “Do Running Mates Matter?: The Influence of Vice Presidential Candidates in Presidential Elections” (University Press of Kansas, 2020). The introductory chapter is available here.