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Now that it’s clear that Joe Biden will be the Democratic presidential nominee in 2020, a lot of the chatter has turned to who he will choose as his running mate. Pundits and political operatives are busy lobbying for their pick, while polls are being conducted and lists of candidates examined.

But the question that is seldom addressed is whether the choice of a running mate really matters. According to political scientists Christopher Devine and Kyle Kopko, not in the way that most people assume.

[T]he research laid out in our latest book on presidential running mates shows vice presidential selection generally has little direct effect on voters… What may be surprising is that running mates usually fail to deliver votes among targeted geographic or demographic groups.

A vice-presidential candidate is not likely to help Biden win a state or garner votes from a particular demographic group. According to Devine and Kopko, there’s only one way that the choice might actually matter.

Our research shows running mates matter, above all else, by shaping how voters view the presidential candidate who selects them… Ultimately, Biden’s vice presidential selection is important—electorally speaking—because of what it says about him.

The best example of how the choice of a running mate mattered was when John McCain chose Sarah Palin in 2008. While the Tea Partiers loved it, the decision reflected very badly on McCain’s judgement and came back to haunt those who made the recommendation. So perhaps the most powerful way that a vice-presidential pick can affect a campaign is when the candidate chooses badly.

Biden has laid out a few criteria that will guide his decision. His running mate will be:

a woman, able to step into the job of being president from day one, someone who agrees with him in principle and philosophically, someone who can make up for his weaknesses, someone he can trust, and someone who will tell him the truth, but will always have his back.

No one knows better than Joe Biden what it takes to be a successful vice-president and he has a lot of truly remarkable candidates to chose from. So he’s not likely to make a big mistake.

With all of that said, there is one way in which this choice of a running mate could differ from historical precedents. Biden has hinted that he might only serve one term. We’ll have to wait and see if that really happens. But if it does, his vice-presidential pick would almost certainly become the presumptive presidential nominee in just four years. That raises the stakes tremendously and is the ultimate reason why Biden’s choice of a running mate matters.