But a good running mate has to do two things: help you win the White House and be good company once you win it. Warren doesn’t pass these two tests.

People think Warren would make a good V.P. because, with Sanders still in the race, Clinton risks losing the support of the left. But that will change once Sanders withdraws. Suddenly, the alternative is Trump. No one thinks this is the 2000 election, when people were lulled into believing that there was minimal difference between George W. Bush and Al Gore. Republicans talk about how enmity toward Clinton will help unify their party, but they’ve got nothing on their own candidate, who will bring millions of Sanders Democrats into line.

The threat of Trump will take care of mobilizing turnout among the Democratic base. So Clinton must subsequently focus on winning over the many Ohioans and Floridians with more conservative social views who could defect to Trump. And as much as Warren’s economic populism might resonate with these voters, many of her other positions, which are as much to the left as those of Clinton, or more, will not.

Second, there is social science. People have gone back and forth on whether having two women on a ticket would be a plus or minus. No one can really say, of course, but some literature that we have on the matter suggests it could, lamentably, be a minus—and in a more complicated way than you might think. When it comes to triggering sexism, research indicates, “two” seems to be a potent number, more so than “one” and more so than “three.” A 2008 study conducted by organizational scholars Denise Lewin Loyd, Judith B. White, and Mary Kern found that being part of a “minority duo” makes those around you far more likely to revert to stereotypical thinking about you. For example, if you’re the sole woman on a board and express a set of views, your colleagues are likely to consider your arguments without much bias. On the other hand, the study shows, if you’re one of two women on a board and both of you are in agreement, then your colleagues are much likelier to attribute those beliefs to the coincidence of your shared sex. “Being a member of a minority duo can emphasize your minority status, and people unconsciously react to that,” Loyd told me. (Let me stress: Loyd was explaining her study, not offering political advice.)

Not surprisingly, perhaps, a 2015 paper by psychologist Kawon Kim found that female duos in such positions tend to feel pressure to differentiate themselves, in order to prove the sexists wrong. If this is a reliable dynamic, then it means that Clinton is seen by voters first and foremost as a Democratic presidential candidate, and not simply a female. But if she were to pick Warren as a running mate, gender could start to color many people’s views much more. That isn’t fair, nor is it fair that manliness tends to be associated with safety, but the world isn’t fair. Currently, Clinton is viewed as plenty tough, which, of course, she is, but an all-female ticket risks ceding that turf to Trump.

Finally, there is the consideration of rapport, which is necessary both during and after the run for office. John McCain and Sarah Palin were mismatched, and everyone could see it. The same went for John Kerry and John Edwards, if in less extreme form. Warren and Clinton also seem mismatched. Warren is a Senate show horse, while Clinton thinks of herself as a Senate workhorse. Warren’s fans love how she grills people testifying before her committee, but many of Warren’s colleagues, not to mention Senate witnesses, grind their teeth over what they see as grandstanding and a Ted Cruz–like tendency to use people as props.