Every presidential candidate claims to be thinking first and foremost about their running mate’s suitability to assume the presidency, but history strongly suggests that few actually behave this way.

And yet there's a good reason this is what everyone pretends they’re doing in selecting a running mate: Ability to serve as an effective president is genuinely by far the most important quality to seek out in a VP.

All of the commentary I've read thus far on Hillary Clinton’s VP search assumes that she’ll continue with the historical trend and choose someone who fits some narrow set of short-term political needs. But she ought to consider taking a break with precedent and seriously meditating on the question, "Who do I want to be my successor?" The answer to that question is the person she should pick.

The stain on Lincoln’s legacy

It’s sobering to note that while Abraham Lincoln is generally regarded as the finest president in American history, his successor Andrew Johnson is one of the worst, if not the worst.

Yet when we ask how the execrable Johnson came to occupy the highest office in the land, the answer is Lincoln. It’s not that Lincoln thought Johnson would be a great president, but he did what most candidates do and picked a VP without giving much consideration to his potential service in the White House at all. Johnson was a Democrat and a Southerner from Tennessee, and a hardcore white supremacist, but he supported the idea of war to preserve the Union. So Lincoln dumped Hannibal Hamlin in 1864 and added Johnson to the ticket to broaden his electoral base of support in border states.

It didn't particularly work — Lincoln lost Kentucky in a landslide — but it also proved to be totally unnecessary, as the election turned out to not be very close.

The problem arose when Lincoln was shot and killed, which brought a white supremacist Democratic Southerner to power at just the moment when anti-slavery Republicans had triumphed in America’s bloodiest war.

Veeps ascend to the presidency frequently

The good news is not every VP who ends up in the White House turns out to be as disastrous as Johnson. Harry Truman was an effective president, and Lyndon Johnson had enormous achievements alongside the significant disaster of Vietnam.

But even when succession works out well, it tends to do so more or less by accident. JFK didn’t put Johnson on the ticket because he thought he would be an effective advocate for civil rights and the welfare state, and FDR certainly didn’t pick Truman because he had confidence in his foreign policy judgment. Kennedy was very narrowly seeking Southern electoral votes (which at least worked — he carried a number of states narrowly that an all-Northern ticket probably would have lost), and Roosevelt barely knew Truman and didn’t include him in important wartime councils.

There was nothing disastrous about Teddy Roosevelt’s accession to the presidency after William McKinley’s assassination, but it did represent an unplanned shift in the nation’s ideological trajectory. A more obscure example comes from John Tyler, who so thoroughly betrayed the ideological proclivities of the actually elected president William Henry Harrison that he ended up being in effect kicked out of the Whig Party. Millard Fillmore, the other Whig veep turned president, betrayed his predecessor Zachary Taylor’s anti-slavery policies by signing the Fugitive Slave Act.

Beyond deaths in office, it's common for vice presidents to go on to secure their party’s presidential nomination — as John Adams, Richard Nixon, Hubert Humphrey, Walter Mondale, George H.W. Bush, and Al Gore did.

The main thing Clinton ought to keep in mind about whoever she picks is that even if she stays perfectly healthy throughout a four- or eight-year term of office, the odds that her pick will be the Democrats’ nominee in 2024 are quite high. She should pick the person she wants to have that job.

Make an honest pick

I wouldn't say that these considerations militate for or against a specific person on her rumored shortlist. Taking succession seriously means not being afraid to tap someone young and "unqualified" like Julián Castro or Eric Garcetti if she genuinely believes he’s a talented person whom the country needs in higher office. But it also means not being afraid to tap someone "boring" like Tim Kaine if she genuinely believes the right person to follow her as party leader is someone who blends into the crowd a bit.

What it does mean is that you pick a populist like Sherrod Brown or an Elizabeth Warren if and only if you actually want the party to shift in a more populist direction.

If Clinton does want to do this, the VP selection is an excellent way to make it happen. But picking a populist VP just as a sop to the populist wing of the party with no intention of actually embracing that approach opens the door to a world of trouble.

The bottom line is that even though VP picks are normally made for narrow political reasons, it’s actually very rare for the VP to give the ticket an important boost. By contrast, it’s fairly common for the VP to become president or at least a major party nominee. The smart move is to leave the crass politics for a decision where the substantive stakes are lower and the political stakes are higher.

The contrast with Trump is important

Of course, many of the same considerations apply to Donald Trump, with the added proviso that he would also benefit from a running mate who, unlike Trump, has at last some level of knowledge of or interest in public policy matters.

But pleading with Trump to make a responsible choice is like asking the sun not to set in the evening. He’s going to do what he wants to do with no regard for larger obligations.

This contrast, in turn, between Trumpian irresponsibility and Clintonian deep engagement with governance is one of the biggest points of political contrast between the two nominees. Clinton ought to own this and accept that while she’ll probably never be the most exciting or personally charismatic politician in America — and no VP pick will change that — she can certainly be the most serious and substantive nominee in the 2016 race. That means actually doing what everyone says they do and picking a VP who is meant to be a successor: nothing more, nothing less.

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