Current events are turning a traditional part of the United States presidential election campaign into an intriguing contest.

Democratic nominee Joe Biden, 77, has pledged to pick his running mate by August to go up against President Donald Trump, 74, and Vice-President Mike Pence, 61.

A vice-presidential pick should be qualified to step in if required and add value in some way.

In 2008, Biden brought experience, familiarity and reassurance to former President Barack Obama, then a first-term senator. In 2016, Pence was the solid, establishment face bolstering a businessman populist.

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At a basic level, the choice should do no harm. Republican John McCain's choice of Sarah Palin in 2008 backfired when she struggled with the role and became fodder for comedians.

But the deputy should also plug weaknesses. Democrat Hillary Clinton's pick of Tim Kaine in 2016 seemed a safe bet for governing but did not help her get over the line. A higher turnout of black voters in swing states could have made the difference.

This time, the choice is particularly interesting.

Biden would be the oldest president elected, has pledged to choose a woman and has hinted he might serve only a single term if elected.

That immediately has an impact on who might be more likely to be picked. If Biden were 20 years younger, for instance, up-and-coming and less experienced candidates would have a better chance. What is more likely is that the running mate will be at least in her 50s and will have senator, governor or congresswoman on her CV.

The former Vice-President's pick would need to be ready and have the authority to step in from the start but also would have to be a potential successor in four years.

Credibility, policy chops, charisma and popularity will matter – in this case, it should be easy to imagine the No 2 as No 1. An appealing vice-president would have an advantage over competitors in 2024 through status, name recognition and experience, just as Biden did this year.

The strange nature of 2020 is also having an impact on who Biden is likely to choose.

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The coronavirus pandemic and economic collapse were having an outsized effect on black and brown Americans even before the death of Minnesota man George Floyd in police custody sparked protests around the world.

Biden already owed his primary win to strong support from African Americans. Black voters are a crucial part of the Democratic coalition.

A CBS poll in May found that the vice-presidential pick was more important to black voters than whites in deciding whether to vote for Biden and that African Americans would prefer a black running mate. And only 75 per cent of black voters said they would definitely vote in November.

With all that has happened in the past three-and-a-half months, an African American running mate seems likely.

A black woman on the ticket would be historic, create excitement and provide a neat comparison with the Obama-Biden combo.

A ticket headed by two white people would look like the wrong fit for a diverse party fighting an election in which racial justice reform will be an important part and against an opponent who has a long record of fanning divisions.

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Biden would appear to need a deputy who can speak knowledgeably about policing, the legal system and the challenges faced by minority communities – and then run that brief in government.

If that is the case, Senator Kamala Harris, 55, would appear the leading candidate. She is the only African American woman who is a current senator or governor and is a former presidential candidate.

Her past as San Francisco district attorney and California attorney-general was controversial with left-wing activists in the Democratic primary who preferred Senator Bernie Sanders and Senator Elizabeth Warren. A Harris pick would not win over progressives who refer to her as "Kamala the cop".

As attorney-general, Harris brought in training on bias for police. But she did not support a bill requiring outside investigations of police shootings - although she now as a senator backs more independent oversight and accountability protocols for police departments. She also did not investigate the killing of a man shot by officers in 2015.

Floyd's killing in Minneapolis police custody has probably ended the chances of Minnesota senator and former prosecutor Amy Klobuchar being picked. But Harris has confidently grasped the moment and been at the forefront of policy efforts in Congress on policing reform. She has spoken about what reimagined policing could look like. A new YouGov poll shows her popularity with black voters has surged.

Harris struggled in the primary to define what her own candidacy was about and wobbled between liberal and centrist positions. But articulating a party policy platform for someone else and arguing against Trump as part of a different team is more straightforward. Biden himself had past unsuccessful presidential runs.

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Harris and Warren, 70, seem to have the strongest cases as campaigners with name recognition, appeal to voters and potential as governing partners. Warren has focused on prescriptions for America's systemic ills. She may be more suited to a weighty Cabinet post.

Other reported contenders being discussed include Florida Congresswoman Val Demings, 63, former national security adviser and UN ambassador Susan Rice, 55, and New Mexico Governor Michelle Lujan Grisham, 60, a Hispanic former congresswoman.

Rice, though highly experienced in government and foreign affairs, has never held an elected position and has tended to be a magnet for Republican attacks in the past. Demings, a former Orlando police chief, could have potential problems from her past career.

Atlanta Mayor Keisha Lance Bottoms, 50, and Michigan Governor Gretchen Whitmer, 48, have been in the thick of the crises this year but may be considered too inexperienced for such a high-level job. That could also put paid to the chances of former Georgia legislator Stacey Abrams, 46.

If Biden is prepared to take a punt on someone with promise, it could be Bottoms. She is a Biden loyalist and an impressive yet down to earth communicator.

The YouGov poll puts Warren as the favoured candidate at 30 per cent, with Harris second on 24 per cent.

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Among black voters, Harris was in front with 25 per cent, and Warren with 15 per cent was fourth behind Abrams on 22 per cent and Bottoms on 16 per cent.

Just last month, Warren was favoured by 41 per cent of black voters and Harris drew 23 per cent. Since April, Harris' favourability rating with black voters has risen from 40 per cent to 52 per cent in YouGov's tracking poll.

Biden will have a lot to weigh up in making his decision but the most likely answer seems the most obvious.