Walter Mondale and George H.W. Bush were home runs. Spiro Agnew and Thomas Eagleton were strikeouts. Now, it’s Hillary Clinton’s turn to name her running mate. It’s a position with paradoxical job demands: Be an energetic attack dog on the campaign trail, but if your side wins, morph into a pliant presidential minion.

And then there’s the maddening near-miss quality of the vice presidency, described vividly by the first person who held the office. “In this I am nothing,” said John Adams. “But I may be everything.”

In modern times, the veep’s job has become much more than “nothing.” Even so, in the last 100 years, only five vice presidents have ascended directly to the Oval Office, three upon the death of a president and one because of resignation. Experience as a former presidential primary candidate doesn’t help your chances of being picked, either. Joe Biden was only the third unsuccessful presidential candidate in the last 50 years to be tapped by the party nominee—and he finished back in the pack both times he ran for president. Finishing a close second in the primaries, as a certain Vermont senator did this season, may make it even worse.

So history suggests that Bernie Sanders is not Clinton’s likely choice. But who should she pick—and why?

News accounts have floated the names of possible running mates, focusing on their personal pros and cons. I also canvassed several prominent Democrats to see what names they are hearing.

The list is composed of six members of the U.S. Senate, including both Virginia senators, Tim Kaine and Mark Warner, along with Elizabeth Warren (Massachusetts), Sherrod Brown (Ohio), Cory Booker (New Jersey), and Amy Klobuchar (Minnesota); three members of President Obama’s cabinet -- Julian Castro, Thomas E. Perez, and Tom Vilsack; and two House members, Reps. Tim Ryan and Xavier Becerra.

Three former White House aides also mentioned three different “long-shot” candidates: Colorado Gov. John Hickenlooper, former Massachusetts Gov. Deval Patrick, and Leon Panetta, the veteran California Democrat who has been a congressman, White House budget director, defense secretary and CIA director.

Assessing the pros and cons of each person on the so-called “short list” is traditionally how the running-mate topic is broached. But there is another way to look at this process: What does Clinton want from her running mate?

First Principle—Do No Harm: Democrats view Donald Trump as a kind of one-man circular firing squad and they just want to stay out of his way. This theory argues for a vanilla pick: someone whose main qualification is that they don’t have skeletons in their closet, won’t say anything stupid, and are acceptable to most Democrats and a good number of independents. Kaine fits the bill here, as does Panetta. To a lesser degree so do Ohio’s Ryan, Patrick, and Klobuchar.

Kaine checks other boxes in the identity politics-obsessed Democratic Party: He’s Roman Catholic, speaks fluent Spanish, and is personally loyal to the Clintons, a factor that carries perhaps undue influence with Bill and Hillary.

“Balancing” the Ticket: Ticket balance was once a very serious business, especially for Democrats. A northern state candidate was paired with a slave state senator, a liberal with a conservative, a governor with a senator, that sort of thing.

Modern ticket balancing is subtler: George H.W. Bush was a “Greatest Generation” war hero who tapped baby boomer Dan Quayle, presumably to appeal to younger voters and women. Famous family man Al Gore — whose wife Tipper had challenged the music industry over kid-unfriendly lyrics — was selected by Bill Clinton after his campaign was nearly derailed by what it called “bimbo eruptions.” Dick Cheney was chosen by George W. Bush to assuage concerns that Bush had too little foreign policy experience.

But Walter Mondale’s selection of Geraldine Ferraro and Barack Obama’s ascension in 2008 expanded the notion of ticket balancing. So who does Hillary pick if she wants to push at glass ceilings?

Kaine doesn’t do much for her on this score. Neither do Warner, Vilsack, or Panetta. Sens. Warren and Klobuchar, however, might solidify the Democrats’ advantage among female voters. Warren, as well as fellow economic progressive Sherrod Brown, might also neutralize lingering resentment among young Sanders primary voters still feeling “the Bern.” African-Americans Cory Booker and Deval Patrick could rekindle the pro-Obama passion in the black community that helped Democrats in 2008 and 2012. Castro, Becerra, Perez would do the same with Hispanics.

Presidents Are Mortal: Eight U.S. presidents have died in office. I suspect that Hillary, who will turn 70 next year, has considered this factor—if, for no other reason, than that her husband gave serious thought to the issue when he was nominated.

In Al Gore, Bill Clinton tapped someone close to his own age, who hailed from the neighboring state, and was in sync with him ideologically. Clinton, whose father died before he was born, picked someone he believed could carry on his policies in the event of a tragedy—as he told friends and even some journalists.

So who does Hillary believe would be a faithful steward of her policies? That’s hard to know, but I’d say that both Virginia senators fit that mold, as do Klobuchar and Vilsack. Then again, everyone on the Clinton’s short list has demonstrated personal loyalty to the Clintons—or they would not be under consideration.

Help Governing: Even if Mrs. Clinton remains robust and healthy for the next four years, the vice presidency has evolved to the point where nominees are looking for a partner in their new endeavor, a sort of super-senior adviser whose judgment and temperament they trust.

This trait might be especially important in a second Clinton White House, expressly because of the long shadow cast by Bill Clinton himself. Who could be a counterweight to him? Panetta is the obvious answer, but not the only one. Kaine, who once ran the Democratic National Committee, is another.

Positioning the Party for the Future: The obvious problem with Panetta is that he turned 78 last Tuesday. If one sees the vice presidency as a steppingstone, Leon is not your man. Neither is Warren (67) or Vilsack (65), while Brown (63) and Warner (61) are on the cusp.

Warner merits consideration because of the Trump factor. Virginia’s senior senator, a centrist Democrat who made a lot of money in the private sector, is unabashedly pro-business and a kind of walking, talking rebuttal to the Republican argument—and it’s not a specious one—that Democrats say they love jobs while seeming to hate employers.

Last year at this time, I would have told you it wouldn’t be Kaine or Warner, but instead Julian Castro. Can you imagine the enthusiasm he’d generate among Mexican-Americans still bristling at Trump’s talk about building a wall and Mexican “rapists”?

Then again, she doesn’t need a Latino running mate to gin up that kind of enthusiasm against Trump—any more than she needs another woman to drive home the point that she’d be the first female president in U.S. history. So my guess? Viva Tim Kaine!