2020 election explained: How and when is the VP chosen?

Vetting and chemistry are important factors when considering running mate

With the Democratic primary effectively finished, the next milestone moment in the 2020 US election will be how former Vice President Joe Biden, the presumptive candidate to take on Donald Trump in November, selects his running mate.

Running mate selections have historically taken place in the spring prior to the general election. The early start time allows for campaigns to select and vet a roster of potential candidates.

Mr Biden has indicated that he'll be making his running mate selection sometime next week, and is choosing from a shortlist for someone with whom he is "simpatico."

Vetting is the process of rigorously examining a political candidate's public and personal history. The practice is done to prevent the catastrophic derailment of a political campaign due to the revelation of some scandalous or otherwise damaging information regarding a candidate.

Vice presidential candidates are generally selected as a way to bolster the presidential candidate's perceived gaps in appeal or experience.

John McCain's campaign picked Sarah Palin in 2008 in part because she appealed to the growing populist Republican base, to which Mr McCain himself - long considered a respectable statesman by the Republican and Democratic elite - was less attuned.

Likewise, Mr Biden was selected to run alongside President Barack Obama because Mr Obama was criticised for lacking legislative experience and for being a radical. Mr Biden had decades of experience as a legislator and was regarded as a more conservative presence in the campaign.

In an inverse of 2008, President Donald Trump had no problem whipping populist Republicans into a loyal army of supporters, but his campaign worried that evangelical Christian voters and the Republican establishment wouldn't feel comfortable supporting him. As a result, Mike Pence - an outspoken conservative Christian with respect among the establishment Republicans - was chosen to bridge the perceived gap.

Picking a vice president is more than simply finding an allied politician who shores up the weak spots in a presidential candidate's appeal; there must be convincing chemistry and respect between the individuals. After all, the president and vice president may have to govern together for - barring a single term presidency - the better part of a decade.

"It may be the most important decision they make -- or at least one of the most important of their presidency -- and it's the one thing that they basically control," former vice president Walter Mondale told ABC News in the run-up to the 2008 presidential election. ""But it is risky, because if you make a mistake, a serious mistake, it's like a bad marriage. Only you can't get a divorce."

While Mr Mondale is mostly correct, it is possible for a sitting president to switch their vice president during an election year.

President Franklin Roosevelt had three different vice presidents across his four terms as president, and both President James Madison and President Thomas Jefferson changed their vice presidents between terms.

In a recent Wall Street Journal op-ed, a supporter of Mr Trump who identifies as a Democrat made the argument that Mr Trump should install Nikki Haley, his ambassador to the United Nations and the former governor of South Carolina, as his new vice president, arguing that she would help him win over "suburban women."

A CNN Democratic strategist Paul Begala went so far as to say the vice president swap was a "certainty," and claimed that Mr Trump would announce Ms Haley's ascension on 16 July, the date the Democratic nominee gives their acceptance address.

Whether or not that claim is true, what is factual is that Mr Trump can change his vice president if he so desires.

In addition to compensating for a candidate's weaknesses, VP picks are also often used to send a message to the party's supporters.

Mr Biden has pledged to select a woman as his running mate, which is likely an attempt to quash criticisms that the party has once again selected an elderly white man as its representative, rather than a woman.

According to Mr Biden's campaign, the vetting process for his possible running mates began at the beginning of May and is expected to last until sometime in July.