Biden's choice of running mate matters, but not for the reasons you may think

Now that Joe Biden is the Democratic Party's presumptive presidential nominee, he must prepare for what previous nominees have called their "first presidential act" — choosing a running mate. The choice matters, but not for the reasons you might think. It will help voters envision what a Biden presidency would look like.

Biden, of course, is no stranger to this process. He was Barack Obama's choice in 2008 and served two terms as vice president. Now it's his turn to choose a running mate, a potential partner on the campaign trail and in the White House.

It's a big decision, especially for a 77-year-old would-be president who voters may fear won't be able to finish his term if elected. Clearly, the choice is on Biden's mind — he made headlines at the most recent Democratic debate by announcing that he would pick a woman to be his vice president.

Presidential candidates typically say that their choice of a running mate comes down to one fundamental criterion: Is the person qualified to take over as president, if necessary?

In private, presidential candidates often say that they also expect the running mate to help them win. Most often, they speculate about a potential home-state advantage. Or that the running mate may deliver a key demographic group, such as women or people of color. More broadly, a popular running mate could help the ticket win by attracting positive press coverage and votes in November.

But the research laid out in our latest book on presidential running mates shows vice presidential selection generally has little direct effect on voters. While most voters say that the choice of a running mate will be important in deciding their vote, few can recall a time when it actually has changed their vote.

And that makes sense. After all, presidents are far more powerful than vice presidents once in office. Why choose a suboptimal president just to elect your preferred vice president? Sure enough, our analysis of survey data from the 1968-2016 American National Election Studies shows that presidential voting mostly comes down to how people feel about the person at the top of the ticket.

Nor did conservatives or evangelical Christians become more willing to vote for the Republican ticket in 2008 following the selection of Palin, who identified with both groups. Although Donald Trump won in 2016, his selection of Mike Pence as his running mate also did not cause more conservatives or evangelicals to vote for him.

Our research shows running mates matter, above all else, by shaping how voters view the presidential candidate who selects them. Running mates indirectly influence voter choice by changing perceptions of the presidential candidate — which, in turn, changes votes.

In the 2008 presidential election, for example, there was reason to question Palin's readiness to serve as president. Biden, however, brought needed experience to the ticket headed by then-freshman Sen. Barack Obama. In both cases, voters noticed. Voters who doubted Palin's experience were more likely to doubt John McCain's judgment — and less likely to vote for the Republican ticket. They also were more likely to believe that McCain was too old to be president.

Conversely, voters who thought Biden had the right experience to be president also viewed Obama's judgment more favorably and were less likely to think that he was too young to be president. As a result, they were more likely to vote for the Democratic ticket.

The lesson for Biden? In choosing a running mate, he must pick a woman (given his recent pledge) who is clearly qualified to be president of the United States — and not, as McCain did, make a desperate gamble on an electoral "game changer" who might undermine voters' confidence in his ability to lead.

Biden can, however, mitigate voter concerns about his biggest purported weaknesses, which include his age and Washington-insider status, by picking someone who is relatively young and yet clearly has the experience to be president, if need be. Think U.S. Sens. Kamala Harris or Amy Klobuchar.

Biden also might shore up his progressive credentials among the party base and disaffected Bernie Sanders supporters by choosing someone clearly to his ideological left, such as Sen. Elizabeth Warren.

In each case, Biden would be shaping voters' perceptions of him as a prospective president, not persuading them to cast a vote for vice president. That's a more realistic conception of the running mate's influence than what typical parlor game analyses would suggest.

Ultimately, Biden's vice presidential selection is important — electorally speaking — because of what it says about him. Will he make responsible decisions in office, not just about policy, but about the people who will surround him? As president, will he put the national interest ahead of his political interest? Will he unite and grow the Democratic Party? Does he value diversity and inclusiveness?

Biden's first presidential act — choosing a running mate — will help voters to decide whether he deserves to be president. That's why it matters.