Hillary Clinton is in the process of whittling down her vice presidential shortlist. But if she's looking to truly rout Trump in November, she ought to consider one old hand who is almost never mentioned: Joe Biden.

When evaluating potential running mates, campaigns look for someone with maximal upside and minimal downside. But the names on Clinton's shortlist creating the most excitement also have significant downsides. Cabinet members like Housing and Urban Development Secretary Julian Castro and Labor Secretary Tom Perez would bring youth and diversity to the ticket, but they are thin on campaigning and governing experience. Clinton won't want to risk picking someone who can be painted as unready to step into the presidency. And picking a high-profile senator like Elizabeth Warren, Sherrod Brown, or Cory Booker jeopardizes the Democrats' chance to retake the Senate. Because of the Democrats' abysmal state level performance in recent years, a Republican governor would get to name their immediate replacement.

Rumored frontrunner Sen. Tim Kaine (D-Va.) is an experienced, qualified pick, but may not generate much added excitement for the ticket. Yes, he's from a swing state run by Democratic governor and Clinton confidante Terry McAuliffe, who would pick his Senate replacement. And while hardly a household name, Kaine has a solid vice president's resume as a senator and former governor. But Kaine will do little to rouse the electorate, either for good or ill. He'd be the Merrick Garland of vice presidential picks: a past runner-up who would be a totally serviceable, inoffensive centrist pick.

But there's another option that is virtually risk-free and carries more upside: Joe Biden. The current vice president is universally known, vetted, and unquestionably fit to serve. As America's affable "Uncle Joe," he is viewed positively by 51 percent of the country and unfavorably by just 36 percent. He's absolutely beloved by Democrats, helping to shore up the Sanders-Warren wing of the party. And he has already proven to be an able complement to a candidate who, like Clinton, sometimes struggles to connect viscerally and emotionally with voters. He also appears to absolutely love being vice president, and is good at the job.

Picking a sitting vice president would not be historically unprecedented. Unlike the presidency, there is no constitutional limit on how many terms a vice president can serve. In fact, both George Clinton and John C. Calhoun served as veep for two different presidents: Clinton for Presidents Thomas Jefferson and James Madison, and Calhoun for John Quincy Adams and Andrew Jackson.

Why might Clinton resist picking Biden? There are three principal reasons: Biden's age, his occasionally strained relationship with Clinton, and his obvious place in the Washington establishment. Biden will turn 74 in November, which is fairly old for a vice president, particularly given Clinton's own age. But at 70, Donald Trump won't exactly offer youthful vitality, either. Moreover, keeping Biden for at least one term would give rising Democratic figures four years to season into stronger veep material before 2020.

The relationship between Biden and Clinton has long been a complicated rivalry. While Biden anguished over whether to run for president last fall, the two jockeyed over which was the rightful heir to Obama's legacy. Tensions continue to linger. When he finally decided against running, Biden took several veiled shots at Clinton from the White House Rose Garden. Biden also recently claimed to have "strongly opposed" intervening in Libya in 2011, putting him at odds with both Clinton and Obama.

But if Obama could peg Clinton to serve as his secretary of state after the fierce 2008 primary battle, Clinton can let bygones be bygones with Biden. Biden endorsed Clinton last month, saying, "God willing, [the next president] will be Secretary Clinton." By all accounts, he's ready and eager to campaign to beat Trump. There's no reason why Clinton and Biden can't paper over their differences to team up.

Conventional wisdom has it that after two terms in the White House, a party's nominee must create distance from the outgoing administration. But as the 2016 campaign circus rolled on, Obama's approval ratings steadily crept up, and 56 percent of the country now approves of his presidency. There's no need for Clinton to run from Obama's popularity, and with her pragmatic approach to center-left policy change, she has already positioned herself as a continuation of his administration.

Certainly, Clinton wants to define her own presidency, and not just be Obama's third term. So she may hesitate at the idea of keeping his vice president.

But Biden would reinforce Clinton's strongest claim to the Oval Office, doubling down on unparalleled experience while adding a warm blue-collar familiarity. As Democrats look to assemble a November landslide, a Clinton-Biden ticket would heighten this election's core contrast: a choice between being governed by the steady hand of the adults on the Democratic side, or by the unserious, unpredictable child put forward by the GOP.