Correction: An earlier version of this article incorrectly stated that John Edwards was a native of North Carolina. He was born in South Carolina. The version below has been corrected.

1. Being the vice presidential nominee is a steppingstone to the presidency.

This is true — but only if your ticket wins. Fourteen vice presidents have become president. Five were elected in their own right, eight ascended to the office when the president died of natural causes or assassination, and one moved up when the president resigned. The odds of a vice president becoming president are about 1 in 3.

But the odds are far worse for the vice presidential nominee on a losing ticket. Only one, Franklin Roosevelt, who was James Cox’s vice presidential choice in 1920, went on to be elected president — and that wasn’t until 12 years later.

In fact, only one other losing vice presidential nominee later won his party’s presidential nod: Bob Dole, Gerald Ford’s running mate in 1976. And Dole did not become the Republican presidential nominee until his third try, when he lost to President Bill Clinton in 1996.

Being the losing vice presidential candidate can end a promising political career, though not all have such a comedown as Palin, who later resigned as governor, or John Edwards, who’s been tarnished by a high-profile affair and a criminal trial. Still, the prospects for a losing vice presidential pick are glum enough that potential nominees might heed Daniel Webster, who declined the offer to run for vice president several times, saying: “I do not propose to be buried until I am dead.”

2. A vice presidential nominee’s most important role is to balance the ticket.

This is true sometimes, as when a young and relatively inexperienced Barack Obama picked 36-year Senate veteran Joe Biden in 2008, or when Washington insider Dick Cheney suggested himself to George W. Bush in 2000 to counterbalance Bush’s perceived lack of gravitas and foreign policy experience.

But one of the most successful contemporary political pairings was of two wonkish, 40-something, white Southerners: Clinton and Al Gore in 1992. Gore’s selection reinforced the message of generational change that Clinton wanted to send as he unseated the last president of the World War II generation, George H.W. Bush.

Clinton and Gore also reported that they had great “chemistry” and enjoyed campaigning together, qualities said to also be high on Romney’s list. But will he find his political soul mate in someone comfortably familiar, such as Sen. Rob Portman of Ohio, or someone younger and ideologically edgier, such as Rep. Paul Ryan of Wisconsin?

3. A vice presidential nominee can carry a key swing state.

This has not happened since 1960, when Lyndon Johnson, ahem, helped John Kennedy win Texas. But that was a time when political machines (or chicanery, in the case of the 1960 voting fraud allegations) could still have a major impact on turnout.

Since then, presidential nominees have generally ignored this consideration, or, if they have tried to heed it, it has not worked. Massachusetts Gov. Michael Dukakis attempted to revive the Kennedy-Johnson “Boston-Austin Axis” in 1988, but having Sen. Lloyd Bentsenon the ticket could not make Texas a Democratic state again. Nor could Edwards deliver North Carolina for John Kerry in 2004.

The near-abandonment of this strategy has been a boon to vice presidential nominees from less populous states, such as Cheney of Wyomingand Palin of Alaska. Sen. John Thune of South Dakota should take heart.

4. A bold running mate choice can energize an otherwise moribund campaign.

This seemed to happen when McCain chose Palin, which electrified the Republican base, but McCain still lost decisively, and Palin’s miscues and combative nature as a self-described “mama grizzly” may have negated any sizzle she gave the ticket.

Presidential candidates do not want to be upstaged by their running mates. They think the election is about them — and it is, as surveys consistently show. History reinforces the polling; selections such as Spiro Agnew and Dan Quayle received heavy media criticism, but each was on the winning side.

But sometimes when the odds are long, a presidential nominee believes he can boost his chances with an exciting vice presidential choice. That was part of Mondale’s thinking in 1984 when he selected Geraldine Ferraro, a congresswoman from New York, to be the first female vice presidential nominee for a major party. But this was after the National Organization for Womendemanded that Mondale select a woman, muting the impact of his choice. Rather than a pioneering leader, Mondale seemed simply to be captive to another special interest. He lost 49 states to Ronald Reagan.

Polls show that the 2012 race should be close, but Romney is running far behindamong Hispanic voters, the nation’s fastest-growing demographic. He could turn to Sen. Marco Rubio of Florida or Nevada Gov. Brian Sandoval to try to change that, but obvious pandering is poor politics — and inexperience on the national stage could cause problems.

5. A nominee’s selection of a running mate reveals how he would govern.

Choosing a running mate is more about politics than governance.

As recently as 1920, when convention delegates ignored Warren Harding’s choice of a running mate and selected Calvin Coolidge instead, presidential nominees had very little to say about who their running mate would be. But in recent decades, presidential nominees have become increasingly engaged in the vetting and selection process because none of them want a repeat of the debacle of 1972, when George McGovern had to drop Thomas Eagleton from the ticket after revelations that Eagleton had undergone electroshock therapy.

Even if not prophetic, the choice says something about the presidential nominee. McCain picking Palin underscored his impulsive streak, while Obama’s selection of Biden suggests that he was aware of his inexperience.

Romney’s pick will reveal something about his nature, too, though it will say more about what he thinks he needs to do to win the election than how he would govern the nation. Despite all the vetting, the key question — will the running mate be up to the rigors and scrutiny of a national campaign? — can’t be answered until after the selection is made.

Scott Farris is the author of “Almost President: The Men Who Lost the Race but Changed the Nation.”

Read more from Outlook:

The most important losers in American politics

Five myths about Marco Rubio

Five myths about swing states

Five myths about conservative voters

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Every presidential election cycle, once a party’s nominee becomes clear, we focus on the veepstakes. Who will Mitt Romney choose? Will he play it safe with a traditional pick, such as Walter Mondale in 1976 or Jack Kemp in 1996? Or take a chance, as John McCain did with Sarah Palin in 2008? Either way, there is no evidence that a vice presidential nominee plays a great part in voters’ decisions. But that doesn’t stop us from overhyping the selection process.