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Joe Biden has started his search for a running mate. He's pledged to pick a woman, but there is still a lot to consider. NPR's Asma Khalid explains some of the calculations.
ASMA KHALID, BYLINE: At a rally in New Hampshire long before Biden was actually the nominee, he outlined two basic criteria for his running mate.
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JOE BIDEN: One, that they are younger than I am - no, I'm not being facetious - and, No. 2, that they are ready on day one to be president of the United States of America.
KHALID: Biden has since elaborated on that list. Here he is on CBS' "The Late Late Show With James Corden" last month.
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BIDEN: You have to be intellectually simpatico. You have to be on the same wavelength. Now, Barack and I never disagreed on the strategy. We sometimes disagreed on tactic.
KHALID: Biden says he is also looking for someone who has strengths that he does not have, which is fairly common. In 2012, the Republican nominee Mitt Romney, who had no experience on Capitol Hill at the time, chose Paul Ryan, who was a leading Republican member of the House. In 2008, John McCain made a competitive calculation when he was running against Barack Obama. Mark Salter was a top aide for McCain.
MARK SALTER: We were facing an opponent who embodied personified change, really, and we needed to be able to make a convincing argument for a change.
KHALID: So the McCain campaign chose Sarah Palin as a running mate.
SALTER: Many people obviously have argued that that choice was a mistake. I won't get into whether it was or whether it wasn't.
KHALID: The conventional rule is that a running mate should do no harm. Kyle Kopko is co-author of the book "Do Running Mates Matter?" His research has found running mates can indirectly affect the election.
KYLE KOPKO: The best example of that in recent times would be someone like Sarah Palin. If a voter viewed Sarah Palin as being unqualified, that ultimately diminished their chances of voting for the Republican ticket.
KHALID: Kopko says that decision affected voters' overall perception of John McCain.
KOPKO: You were more likely then to question his judgment. Maybe he isn't someone who can adequately perform the job of president. Maybe he's too old to be president, and so on and so forth.
KHALID: And Kopko says there is also often an assumption that choosing a candidate from a specific part of the country might win you votes from that region. But...
KOPKO: Our research shows that that very rarely happens and that tends to be really overstated by political commentators.
KHALID: Still, Kopko says selecting a vice president is a presidential candidate's first real decision. So by Biden promising to choose a woman, it sends a message about his priorities. This process, though, has gotten underway as Biden is facing a sexual assault allegation from a former staffer. He firmly denies it, but the allegation still means that some of the women being considered for the job are already in the position of having to defend Biden, which many of them seem willing to do. Still, some Democrats have been encouraging Biden to not just choose any woman.
JIM CLYBURN: I've made it very clear like a lot of other people that it would be great if the choice were a woman of color.
KHALID: That's South Carolina Congressman Jim Clyburn, the highest ranking African American member of the House. But Clyburn says what's most important is actually the chemistry between running mates and how the public perceives that chemistry.
CLYBURN: I think that people see right away if the chemistry is there, so I think you've got to do extensive vetting. You've got to do extensive polling because this is serious business. You don't get a do-over in this.
KHALID: Biden is in the unique position of having held the very job for which he's now trying to hire. And that matters according to Democratic consultant Tad Devine.
TAD DEVINE: I think when you have that perspective of being the vice president, the relationship between yourself and the president is really important to you as a potential president.
KHALID: Devine, though, is skeptical of how much a VP pick genuinely affects the outcome of an election. In 1988, he was Lloyd Bentsen's campaign manager. That year, Bentsen was the Democratic VP nominee, and he was more experienced than his Republican opponent Dan Quayle.
DEVINE: And I saw that even when you have an incredible mismatch of vice presidents on both sides, that that in the end doesn't really affect the outcome of the election that much.
KHALID: But Biden's pick might be different. At 77, it is highly plausible that if elected he would only serve one term. And so the woman he picks wouldn't just be the potential VP; she would likely be the Democratic Party's next standard bearer.
Asma Khalid, NPR News. Transcript provided by NPR, Copyright NPR.