They are the running mates – or, to put it another way, second fiddle to the main show in the U.S. presidential race.

But every four years, political junkies and pundits take part in the parlour game that is "veepstakes": Who is in the running to be a vice-presidential nominee?

This year, with no incumbent president seeking re-election, the Democratic and Republican presumptive presidential nominees must declare their picks.

With the party political conventions a few weeks away – the Republicans are first up (July 18), followed by the Democratic convention (July 25) – talk of vice-presidential picks is at its peak.

On Friday, just days before the Republican convention, Donald Trump made his pick official.

The vice-presidential picks speak volumes about each candidate's judgment and the message they are trying to send to voters. A poorly vetted and unprepared vice-presidential nominee could end up being a drag on the campaign at a time when the general election is about to go into higher gear.

In 2016, two deeply unpopular presidential candidates are poring over lists of names trying to figure out how best to fill the ticket and give their campaigns a jolt.

Picking who will be one heartbeat away from the presidency is no minor task. Here is your guide to the rules of "veepstakes," the names that are in the mix, and whether the enshrined picking of a No. 2 ever moves voters.

1. The Oval Office test

The minimum requirement for any vice-presidential pick is that voters are able to imagine that person stepping into the Oval Office as U.S. president, according to Joel Goldstein, law professor at Saint Louis University and author of The White House Vice Presidency: The Path to Significance.

The U.S. system requires a vice-president to lead the country in case the president is unable to complete his or her term.

Examining the picks over the last 40 years, Prof. Goldstein says the track record is mostly impressive – with picks representing the "top political leaders of their generation." There are some exceptions: Dan Quayle was widely seen as unqualified in 1988, while the same could be said of Sarah Palin in 2008.

"By and large, presidential candidates tend to choose pretty well," said Prof. Goldstein.

2. The 'do no harm' rule

The heart of this rule is simple: Do not pick a vice-presidential candidate who will hurt the presidential nominee.

The act of picking a running mate is the presidential candidate's "first presidential action" and encapsulates his or her values, according to Kyle Kopko, associate professor of political science at Elizabethtown College in Pennsylvania and co-author of The VP Advantage: How Running Mates Influence Home State Voting in Presidential Elections.

A bad pick reflects on the presidential candidate, in large part because voter evaluations of the presidential candidate carry three times the weight compared to evaluations of the vice-presidential candidate, according to Prof. Kopko.

"You want to select someone who's competent, someone who's going to do the job of the president if need be, someone who's going to be a great adviser. But also someone who's not going to overshadow the candidate or be controversial," he said.

3. Pick a candidate who can withstand the bright lights

The whirlwind begins the moment a vice-presidential nominee is announced – sometimes days or weeks ahead of the political convention.

What follows is a slew of sit-down media interviews, a convention speech to write, hitting the campaign trail, and enrolling in debate boot camp for the live vice-presidential debate some time in the autumn.

"The question is: Do you really want to put someone on TV for 90 minutes on that occasion and do you want to put someone opposite Katie Couric, and the equivalents, who is going to just unravel like [Sarah] Palin did," said Prof. Goldstein.

In 2008, Ms. Palin mishandled questions on how her tenure as Alaska governor – and the state's proximity to Russia – enhanced her foreign-policy credentials.

Nine minutes in, Katie Couric asks Sarah Palin about her comments on Russia

4. Pick a candidate who will balance the ticket

By balancing the ticket, political observers mean filling the gaps. For example, presidential candidates that are Washington outsiders tend to pick insiders, explains Prof. Kopko.

In 1952, Republican presidential candidate and Second World War hero Dwight Eisenhower picked Richard Nixon, a younger running mate with deep experience in the U.S. Senate and House of Representatives, he added.

Another way to balance the ticket is to pick a running mate who will fill a gap in expertise – as in the case of George W. Bush picking Dick Cheney in 2000.

"He was sending a message: that even though I've just been the governor of Texas and I don't have any national security credentials, I'm going to have at my side the guy who ran the Defense Department during the [1991] Persian Gulf War and this is going to be the kind of people I'm going to surround myself with," said Prof. Goldstein.

In 2016, there is much talk of picking running mates that will appeal to women and Latino voters – another way of trying to balance the Republican and Democratic presidential tickets by appealing to cross-sections of the electorate.

5. It takes two to make a presidential ticket – chemistry is crucial

Imagine spending the next eight years of your life with someone you can't stand.

It turns out the need for good chemistry may be the single driving factor in how presidential candidates pick their running mates.

"It's highly personal, so the two need to get along. They have to at least respect each other and like each other because they spend a lot of time together and they're best able to compete as a team when they are in sync," said Dianne Bystrom, the director of the Carrie Chapman Catt Center for Women and Politics at Iowa State University.

In 2012, the Republican ticket clicked on the campaign trail during a tryout.

"The story goes that Mitt Romney and Paul Ryan, who are both kind of big thinkers, policy wonks, got along really well and that was a key factor in Mitt Romney's decision," said Prof. Bystrom.

Arguably one the best examples of good chemistry is current President Barack Obama and Vice-President Joe Biden.

"Honestly, that was a pretty wise pick on President Obama's part. Joe Biden has been a tremendous … cheerleader for the administration, someone who's been very supportive," said Prof. Kopko.

6. Don't botch the rollout of your pick

Indiana Senator Dan Quayle was told that he would be the vice-presidential pick in 1988 two hours before George H. W. Bush made the surprise announcement at a packed outdoor New Orleans rally where Mr. Quayle had to fight through the crowd to get on stage with Mr. Bush, explains Prof. Goldstein.

"He was hyperactive. But in his defence he'd just hit the jackpot and physically exerted himself to get up there. So he came through like a high-school cheerleader rather than with any kind of gravitas. But it wasn't entirely his fault," he said.

The Bush campaign then threw Mr. Quayle into a round of media interviews with little preparation about how to handle questions about serving in the National Guard instead of going to fight in Vietnam.

"Serving in the National Guard wasn't treasonous, but it sort of became a feeding frenzy and the Bush people weren't really prepared to defend him. So the initial impression of him was bad," said Prof. Goldstein.

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The challenge facing the presumptive Republican presidential nominee can be boiled down to a single question.

"Who is going to agree to be a VP with Trump? That's a tough job," said Iowa State University's Prof. Bystrom.

To be sure, there are people who want to be his running mate.

But some of the brightest and most dynamic GOP politicians have demurred or flat-out said no because of the bruising nominating contests where Mr. Trump relentlessly ridiculed his rivals with nicknames and schoolyard taunts.

"1 for 41" John Kasich, Ohio's Governor and former U.S. congressman, would give Mr. Trump the Washington insider he wants. But there are more pressing issues: such as Mr. Kasich not yet endorsing Mr. Trump. "Little" Marco Rubio – conservative, Latino and steeped in U.S. foreign policy – would have been a dream running mate, but that bridge was burned long ago.

"[Trump's] really looking for someone with [Washington] government experience and those are the types of people that I think are a little more anxious about what he brings as a candidate because he's very unpredictable," said Prof. Bystrom.

"So he wants this very predictable government-experience type person but he's a very unpredictable candidate," she added.

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The presumptive Democratic presidential nominee has the perfect Washington politician résumé, with roles as first lady, U.S. senator and U.S. secretary of state.

Ms. Clinton does not need to shore up on policy experience because she is a policy wonk, said Prof. Bystrom. "I think she will go with a constituency or electoral-map choice," she added.

What that means is she will likely pick a running mate from a battleground state or someone who appeals to the liberal wing of the party. She could also pick a Latino running mate – and in doing so, make history. But there are drawbacks with each choice.

What's more, there is a wild card in her decision-making: her husband, 42nd president Bill Clinton.

"So with Bill Clinton still being in the picture you're also going to have to take into account, well, if he's going to want an active role, is there going to be a vice-presidential candidate who would also work well with him and not feel like he's overstepping his boundaries or overshadowing the vice-president," Prof. Kopko said.

"So I think that's a really unique dynamic that we haven't seen before in modern presidential elections," he added.

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Do vice presidential picks swing elections?

The short answer is: not really.

The example that is often mentioned is the 1960 presidential election. Putting U.S. Senator Lyndon Johnson on the presidential ticket was an attempt by John F. Kennedy to ensure that Texas – Mr. Johnson’s home state – and other southern states swung in favour of Mr. Kennedy.

“Johnson even calls up Kennedy on the night of the election and tells him: I’m delivering Texas. But whenever we dig into the data … the data just don’t support that claim,” said Prof. Kopko.

“If Johnson had delivered Texas for the Democrats, and the South for that matter, we would expect to see Johnson having very high approval ratings, just wild, over-the-top support for his candidacy. And in many instances, Kennedy was just as popular if not more popular in some of the internal campaign polls,” he added.

John F. Kennedy and Lyndon Johnson on the campaign trail in the 1960 election

In 2016, when presumptive nominees are weighing running mates that could appeal to women and minority voters, the actual evidence that such picks move the needle in a general election is scant, according to Prof. Kopko.

Women voters viewed Democratic vice-presidential candidate Geraldine Ferraro in 1984 and Republican vice-presidential candidate Sarah Palin in 2008 favourably, but that did not translate into votes, he said.

What would be the impact of a Latino vice-presidential candidate? For one, there’s never been such a pick. But Prof. Kopko has a feeling that a Latino vice-presidential candidate would not necessarily deliver more Latino votes.

“If we were to see a much higher percentage of Latino or Hispanic voters voting for Hillary Clinton if she takes a Latino or Hispanic running mate, is it really because of the running mate or is it because there is such a massive difference between how Clinton and Trump approach issues like immigration, like border security, like profiling?” said Prof. Kopko.

“Our conclusion based on the research is that it’s primarily the policy of the presidential candidates that’s driving the vote choices,” he added.

There is, however, one election that offers a telling “what if” scenario.

If Democrat Al Gore had chosen popular New Hampshire governor Jeanne Shaheen – who was on his VP short list – as his running mate in 2000, it is very likely that Mr. Gore would have carried the swing state.

“By our estimate, if Shaheen had been the candidate, the Gore ticket would have won New Hampshire by about a point and a half. That’s a conservative estimate. And that would have been enough to secure a majority in the electoral college for Al Gore, and he would have been president,” said Prof. Kopko.

“Florida would have been totally irrelevant in that situation,” he added.

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