Vice presidents can play an outsized role in the White House beyond the narrow authorities granted by the US Constitution. And as candidates, they can help carry the message of their campaign and boost it.

This year, President Joe Biden Biden is expected to stick with Vice President Kamala Harris as his running mate. Former President Donald Trump is set to announce his VP pick during or shortly before the Republican National Convention next month.

The VP is elected as part of the presidential ticket. So, if people vote for Biden’s re-election as president, they indirectly elect Harris for another four-year term as VP.

As the campaign season heats up, Al Jazeera looks at the VP position in the US and how vice presidential nominees are chosen.

What is the vice president’s role in the constitution?

Christopher Devine, a political science professor at the University of Dayton who has written two books about vice presidential candidates, said the VP has a “very limited set of responsibilities” according to the constitution.

“The VP takes over for the president if something happens – death, resignation or even in case of temporary incapacity. That’s the big one,” Devine told Al Jazeera.

The vice president also casts the tie-breaking vote in the Senate – something that Harris did regularly in the first two years of Biden’s presidency when the chamber was split 50-50 between Republicans and Democrats.

The vice president is also the president of the Senate, but VPs have largely stopped presiding over legislative proceedings – a mostly ceremonial role – in past decades.

What does the VP do beyond official duties?

Nowadays vice presidents act as presidential advisers. They are expected to be the “last person in the room” before the president makes major decisions, Devine said.

“They also play other roles, like being a liaison to Congress, helping to negotiate on legislative matters,” he told Al Jazeera.

“Of course, they have some role in foreign policy – meeting with foreign heads of states or doing diplomatic missions, attending funerals and the like. So there’s lots that vice presidents do. But most of what they do is not required by the constitution.”

Who defines the role of the VP?

Modern US history has seen powerful vice presidents – like Dick Cheney, especially during George W Bush’s first term –  as well as ones who had a low profile and were not seen as pivotal to their administration, such as Mike Pence, Trump’s former VP.

Devine said the president ultimately shapes the role of the vice president.

“It’s really up to the individual presidents. So vice presidents can be used as a great asset to advise the president on important matters. They could also be pushed to the side,” he told Al Jazeera.

Lindsay Chervinsky, a presidential historian, said another factor that determines vice presidents’ level of influence is their own experience and relationships.

For example, Cheney had served as a defence secretary, congressman and White House chief of staff.

“They need to have something that they can bring to the table in a way that allows them to call on their expertise and connections in order to make an impact,” Chervinsky said of vice presidents.

How are VP candidates chosen?

Unlike candidates for president, VP candidates do not run in party primaries. They are chosen after the presidential nominee is determined.

Presidential candidates pick their own running mates who then get the official nomination during their party’s convention.

Devine said candidates usually start with a long list of candidates and then start narrowing it down after rounds of interviews and background checks.

“The vetting process is incredibly invasive because they’re trying to find out if there are any skeletons in the closet that could come out about the vice presidential candidate during the campaign,” Devine said.

“So they ask for tax records, medical records, they do interviews with people who have worked with the candidates, with family members. They want to know everything.”

What do presidential candidates look for in a running mate?

Chervinsky said candidates typically want their VP pick to be a good public speaker to advance the campaign’s message; being an “attack dog” who can effectively rebuke opponents also helps.

“They’re also usually looking for someone who is different from them in some way,” Chervinsky added.

The considerations could include race, age, gender and experience, to help with so-called ticket-balancing, which is meant to broaden the appeal of the campaign.

For example, Biden picked Harris – a Black woman who is considerably younger than him. Barack Obama, a first-term senator at the time, chose Biden, who had been in the Senate for 35 years.

Devine said beyond electoral politics, candidates also look for a capable governing partner.

“They’re looking at who can help win the election, but they’re also looking at long term who can help govern once in office,” he said.

Do VP picks matter in elections?

Both Devine and Chervinsky said VP nominees do not make or break campaigns.

“To the extent they matter, it’s by influencing how people see the presidential candidates’ judgement,” said Devine. “Do they make responsible decisions or not?”

What about Trump’s VP pick?

Trump had fallen out with his former VP after Pence refused to use his ceremonial role of counting the electoral college votes in Congress to reverse Biden’s victory.

“It’s clear that Trump is looking for someone who is loyal to him above all other things, including the constitution,” Chervinsky said.

She added that he may choose a woman or a person of colour as his running mate to appeal to those subsets of voters.

Trump’s rumoured VP picks include Congresswoman Elise Stefanik, Senator Tim Scott, Senator JD Vance and North Dakota Governor Doug Burgum. Scott is the only Black Republican in the US Senate.

“Ideally, he should be looking for someone who could be the responsible governing party partner, but I don’t really think that’s generally how his considerations are made,” Chervinsky said of Trump.