Hillary Clinton, as Secretary of State, greets Sen. Elizabeth Warren (D-Mass.) as they arrive for Sen. John Kerry's (D-Mass.) confirmation hearing before the Senate Foreign Relations Committee to become the next Secretary of State. (Photo by Chip Somodevilla/Getty Images)

As the 2016 race's veepstakes gets into full swing, the buzz over whether Sen. Elizabeth Warren could be Hillary Clinton's running mate has only grown in volume. With the pair meeting Friday morning after Warren issued her endorsement Thursday afternoon, Warren's repeated Donald Trump-bashing may look more and more like an audition for the No. 2 role.

If the chances of such a historic all-female ticket happening still seem quite remote -- and it very well may be -- that's at least in part because a major U.S. political party has never had a woman at the top of its ticket, much less two.

But it's also because having a pair of women take the top leadership roles is not just unusual for presidential campaigns -- it's rare in many other spheres, too.

The most analogous place to look in the U.S. would be gubernatorial races. Over the past twenty years, there have only been five pairs of women who have run for governor and lieutenant governor in the same year, according to Debbie Walsh, the director of the Center for American Women and Politics at Rutgers University. And in only two of those cases -- in New Jersey in 2013 and Kentucky in 1999 -- did the two run as running mates, rather than in separate races. (In some states, governors select their lieutenant governors to run on the same tickets; in others, they run separate races.) None of the five pairs of women was elected.

A less similar place to look are the states that have elected women to hold both senate seats at the same time. That, according to Walsh, has happened four times -- in New Hampshire, California, Maine and Washington. Even if the two obviously don't run together, and one clearly doesn't select the other for the job, it is an example of voters being willing to put two women in two prominent leadership roles, said Walsh in an interview: "Will voters pull the lever for two women? What we saw in those races was yes, they have."

Sen. Elizabeth Warren (D-Mass.) endorsed Hillary Clinton for president on June 9, but the two haven't always seen eye-to-eye. (Peter Stevenson/The Washington Post)

Globally, there are also few parallels. Catherine Reyes-Housholder, a PhD candidate at Cornell University who has written about gender-balanced cabinets, says Violeta Chamorro of Nicaragua governed with a female vice president, Julia de la Cruz Mena Rivera, from 1995 to 1997, but the two did not run together. "What would be unprecedented, again from a Latin American perspective, would be a female presidential candidate naming a female running mate and winning," she said in an email. "That has never happened."

Diana O'Brien, an assistant professor of political science at Indiana University, said in an email that female prime ministers in Western Europe or industrialized nations generally do not appoint female deputy leaders, and couldn't think of "any instance that would exactly parallel" a Clinton-Warren ticket. She notes that in the first cabinet of former Danish Prime Minister Helle Thorning-Schmidt, the deputy prime minister was also a woman, but the two were not from the same party. And in Finland, which has a president who acts as a ceremonial head of state and a prime minister who heads the government, she says, there have been times when both of those roles were held by women, but each female prime minister selected a male deputy leader.

Meanwhile, outside the political sphere and in the largest publicly traded corporations in the United States, it's also exceedingly rare for two women to hold the two top leadership roles. According to the nonprofit research firm Catalyst, there are just 21 S&P 500 companies that have female CEOs. Even if roughly half of these women also hold the chairman's role themselves, just two of them have separate women who also lead the board of directors.

It's an unusual enough arrangement that when leadership consultants Betsy Polk and Maggie Ellis Chotas went to write their book "Power Through Partnership: How Women Lead Better Together," they had trouble finding examples of well-known female partners who've held leadership or entrepreneurial roles together. "What we found was that in contrast to men, who have many celebrated and well-known partnerships, high visibility collaborations between women were hard to find," said Polk in an interview.

Such partnerships may be under the radar, she said, because there's often too much focus on what happens when female partners don't work out -- think of the cat fight stereotype. Or, women may hesitate to promote their work together because they believe they have to meet a higher bar. They may be concerned, Polk said, that "promoting a collaboration with other women is akin to 'we couldn't do it ourselves,' " when in fact, such partnerships "can be very successful."

However unusual it may be, all the focus on whether people would be willing to vote for an all-female ticket is telling. "It's interesting to me, because how many pairs of white men have we had running for president of the United States? And we've never had that moment of 'hmmmm, will Americans pull the lever if there are two white men?' " Walsh, of Rutgers, said. "I would like to think that in 2016, if it’s a strong ticket, it’s a strong ticket." 

Read also:

Why Elizabeth Warren’s attacks could be just as powerful from the sidelines

Why Hillary Clinton’s goal of a half-female Cabinet really matters

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