This is the moment, Vice President Biden. Time to sit down with all these potential running mates, the last survivors in the field, and size them up for yourself.

Your advisers — not to mention an army of my colleagues — have already broken down all the relevant cases for and against this one or that one. Kamala D. Harris raises money. Elizabeth Warren stirs the base. Susan Rice gets you foreign policy. And so on.

Do yourself a favor: Throw it all out the window. All those arguments are worth about as much as a degree from Trump University.

There is only one thing that matters in a vice president, and you won’t find it in a voting record or a quarterly filing.

Your pick has nothing to do with election math. Even if you buy the notion that this year’s vote is all about turnout (and I don’t), there’s no evidence to suggest that a running mate does anything to mobilize voters. And you already have the best turnout generator in American history: His name is Donald Trump.

Fundraising? Irrelevant. You’ll have all the money you need, and in a year when you can’t even travel around to cut the ribbons on shiny new offices, good luck even spending it all.

Ideology? Expertise? Who cares? You’re not some governor who has never left the country except on holiday. (No offense, President Bush.)

You’ve spent a half-century in Washington; eight years in the White House. We know who you are. You don’t need a running mate to fill the gaps.

No, there’s really only one question you need to ask yourself as you sit with all these aspirants for your old job. It’s not a pleasant question, but it is a necessary one.

Who has the emotional intelligence and humility to unite a leaderless country, should it come to that?

This is the one enormous thing we ask of a vice president, in rare moments, that we ask of no other leader in government. Should the worst occur, should the chair behind that big oak desk suddenly be vacant, the nation turns its gaze to someone it has barely considered before and asks that person to get us through.

This has nothing to do with knowing which foreign leaders to call or which bases to put on alert. This is about grace and warmth and political instinct. It’s about an understudy’s innate ability to be firm and decisive, while also understanding that the moment isn’t solely about her.

It’s why Harry S. Truman, the hat salesman from Missouri who inspired little confidence at the outset, is considered one of our greatest presidents. It’s why Gerald Ford, for all his flaws, gets credit for leading the country through an unprecedented and tumultuous moment.

The lack of those qualities is why Lyndon B. Johnson, petty and paranoid, is still the subject of revulsion and dark conspiracy theories in popular culture, despite all his legislative success in the wake of President John F. Kennedy’s death.

Surely you of all people understand this, Mr. Vice President, having held the job under a president whose safety was a constant source of worry. It’s what made you such a strong and reassuring pick — because if there’s one thing people know about Joe Biden, it’s that he’s a decent man who has risen in the face of tragedy before.

Had the worst happened, had you been called on to serve, Americans would have seen a president who shared their grief while reasserting order. Whatever else you achieved as a No. 2 — whatever advice you offered or negotiations you led — was always secondary to that.

I know this is an uncomfortable subject, Mr. Vice President, and probably no one’s in a hurry to raise it with you. We always hope it doesn’t come into play, and usually it doesn’t.

But it’s not as though you don’t know that you’d be the oldest president ever inaugurated. It’s not cruel, but rather squarely in the national interest, to acknowledge that the odds of your being incapacitated in some way are higher than for someone who’s 60.

Which is why all you should ask yourself, as you evaluate each candidate face-to-face, is which of them you could see standing next to your own family, honoring your own legacy, speaking to a shaken nation and soothing a divided Congress.

Who puts raw politics and self-interest aside? Who has enough human perspective to feel unworthy, but also the talent and perseverance to prove otherwise? Who can turn tragedy into opportunity?

The life of the nation turns on such moments, whether we want it to or not.

Don’t pick an orator or a fundraiser or a policy wonk, Mr. Vice President. Pick the best person, and trust the voters to do the rest.

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