While a vice presidential pick has never been solely credited with swinging an election, it can leave an indelible impression about the judgment of a would-be president or help address something that is lacking at the top of the ticket -- whether it be regional appeal, ideological purity or a gap in their resume. Sometimes, it's all of the above.
As presumptive nominees Donald Trump and Hillary Clinton inch closer to make their running-mate choices final and public, speculation has reached a fever pitch. NBC News spoke to renowned presidential historian and NBC News contributor Michael Beschloss to get some clarity about the historical precedents at play.
"In terms of just the first rule of choosing a vice president is ‘do no harm,'" Beschloss said.
Former Democratic presidential nominee Sen. George McGovern learned that lesson the hard way in 1972, with his botched selection of Sen. Thomas Eagleton in the midst of his party's chaotic convention that year.
Already trailing in the polls against popular incumbent President Richard Nixon, McGovern chose the Missouri senator in haste after several other prominent Democrats (including Sen. Ted Kennedy) declined to run alongside him. When asked about potential skeletons in his closet, Eagleton reportedly said he had none.
But days later it was revealed that the he had battled clinical depression for years and had even received electroshock therapy as part of his treatments. After initially backing him, McGovern cut ties with Eagleton, and the White House pounced, arguing that the Democrat had failed his first test as a potential commander-in-chief.
McGovern would go on to lose 49 states in one of the most lopsided general election results in U.S. history. Ever since, the so-called "vetting" process, while sometimes imperfect, has become increasingly crucial for major-party nominees.
"A lot of that goes right back to Eagleton," said Beschloss. "The McGovern people rightly felt that if they had floated the name of Eagleton to the public a week earlier, some of the issues about him and his illness might have come out and they would have been saved from making a nomination that turned out to be disastrous for McGovern."
The lack of modern polling makes it impossible to definitively track the impact or lack thereof of VP picks on a given ticket until arguably the second half of the 20th century, but most recent vice presidential picks can be broken down in terms of the value they brought to the candidate at the top.
Lyndon Johnson has become the gold standard when it comes to a VP selection who was actually capable of delivering raw votes to a party's ticket. The then-Texas lawmaker helped put the Lone Star state in John F. Kennedy's column, and broadened the Massachusetts senator's appeal in the deep South. But today, according to Beschloss, this kind of power is almost non-existent.
"So many many Americans live all different places and they don’t feel as tied to their home state as they did 100 years ago," he said. "For instance, if you had a VP candidate from, let's say Indiana (like Gov. Mike Pence), 100 years ago most people from Indiana would stay there their entire lives and they would feel great pride in an Indiana candidate even if it was not one of their own party. Now, people are more mobile."
Sometimes a VP nominee can help reassure members of a presidential candidate's base when it comes to their ideological purity, or in other cases tempter their extremism. Former President George H.W. Bush performed that role to a certain extent in 1980, when he was tapped by his primary rival Ronald Reagan. The choice of the more moderate Bush was a much a signal to Reagan's fellow Republicans as the electorate at large.
Of course, sometimes a pick meant to appease party-members can backfire, like when in 1976 President Gerald Ford ditched incumbent Nelson Rockefeller for the more conservative Sen. Bob Dole. The Kansas senator was seen as nod to the Reagan wing of the party (which Ford had narrowly defeated in the primaries) and an attempt to regain the president's footing in the Plains states.
But the mood of the electorate didn't suit Dole's hard-charging style, as evidenced by the backlash to his remark, "If we added up the killed and wounded in Democrat wars in this century, it would be ... enough to fill the city of Detroit," in his first and only debate.
And Rep. Paul Ryan's popularity with the GOP base was not enough to boost Mitt Romney to victory in 2012.
Though not as common, sometimes a vice presidential pick is about doubling down on an already established brand. The best recent example would be in 1992, when then former Arkansas Gov. Bill Clinton decided to pick Sen. Al Gore as his VP.
"Al Gore helped bring Tennessee, which was not exactly a pivotal state in the Clinton electoral victory, but you can say that he strengthened the public view of Clinton as young and moderate and innovative and Southern, so he essentially doubled Clinton’s message and strengthened some of his appeal," said Beschloss. "That’s something else a vice presidential candidate can do that you cannot quantify with polls."
Some have speculated that if Trump choose New Jersey Gov. Chris Christie, a politician who is perceived to be very much in the same mold as the real estate developer, it would be another example of this phenomena.
In the last few election cycles, with relatively inexperienced candidates like George W. Bush and Barack Obama, it's become increasingly common to see a VP selection as opportunity to assuage voters by filling in a gap in a contender's area of expertise. In both of their cases, Bush and Obama were lacking in substantial foreign policy experience, and therefore tapped older, establishment figures who would likely not be seeking to supplant them one day, but instead provide trusted counsel.
As a result, both Dick Cheney and Joe Biden have been viewed as two of the most active, engaged and vital vice presidents in history, perhaps permanently changing how the role was initially conceived by the founders. In particular, the purported personal closeness of Biden and Obama is historically unprecedented.
The Hail Mary
While political experts would caution against making the bold, outside-the-box pick because of its high risk, potentially low reward nature (see Eagleton), it still happens, sometimes because a campaign feels they need to something to electrify the voting populace.
That appears to have been at least part of the rationale in 1984, when Walter Mondale selected Rep. Geraldine Ferraro as the first woman to ever headline a major-party ticket. However, after an initial burst of excitement, bad press about her personal finances and a less-than-stellar showing in the VP debate dulled the impact of her historic achievement.
Twenty-four years later, Sen. John McCain faced a similar dilemma after he picked then-Alaska Gov. Sarah Palin to be his running-mate, a choice that very few election watchers were anticipating. A series of now-infamous gaffe-filled interviews and self-inflicted controversies involving Palin appear to have taken a toll.
"I think people who work for John McCain, he wouldn’t say [this], but they would say that Sarah Palin diminished him as a candidate," said Beschloss, citing MSNBC contributor and former McCain aide Nicole Wallace's decision to abstain from voting in 2008, in part because of concerns over Palin.
What makes the 2016 campaign unprecedented, is that both major party candidates are historically unpopular with the electorate at large, and both are viewed as seriously lacking on a range of issues from trust (in Clinton's case) to experience and temperament (in Trump's). Meanwhile, now that the role of the vice president has begun to expand -- they only began having their own debates in 1976 -- there is an expectation that a candidate be ready to assume the top role from day one, should a tragedy occur.
"For most of American history the president had virtually nothing to do with the vice president. The vice president sat in an office on Capitol Hill, the president barely talked to him, and so that was not an issue," said Beschloss.
In the wake of the killing of President John F. Kennedy, the assassination attempt on President Ronald Reagan and recent breaches of Secret Service protection, coupled with a climate where international terrorism is a clear and present danger, voters are not naive about stakes of being in the hot seat should the commander-in-chief be incapacitated.
Still, according to Beschloss, the burden will still be on the top of the ticket to persuade voters to embrace them personally this fall despite their initial reservations.
"There’s not any polling evidence that really shows vice presidents do a lot to change peoples’ perceptions of the human qualities of a presidential candidate," he said. "They really have to do that for themselves."