The calculations that go into choosing a presidential running mate are intricate: Balance age, experience, ideology, demographics – and look for a personal connection – to create the perfect electoral ticket.

In the case of Joe Biden, the 77-year-old presumptive Democratic nominee who will name his vice presidential selection panel this week, the choice of a highly competent partner is particularly important – both because of his advancing years and the ongoing COVID-19 crisis.

Who's Biden looking for?

Biden has spoken extensively about potential running mates, setting out possible criteria. He has said unequivocally that he will pick a woman, and also emphasized that readiness to take the top job will be at the forefront of his considerations.

"The first and most important quality is someone who, if I were to walk away immediately from the office for whatever reason, that they can be president and the public could look at that person and say: 'She is capable of being president of the United States tomorrow,'" he told CBS' Late Late Show last week.

Given Biden is only likely to serve one term, he'd also be picking a potential successor for 2024.

The former vice president will consider the pros and cons of balancing race and ethnicity – around 40 percent of Democratic voters are non-white, and black voters were hugely important to his primary success. Many of his supporters, including influential South Carolina Representative Jim Clyburn, have said they want a black woman on the ticket. 

Age – Biden would be the oldest president ever inaugurated – and ideology – he is on the moderate wing of the party – will also be weighed, as well as whether the pick can help him win votes in the battlegrounds states of the Midwest. And having served as vice president himself, he is also looking for someone he can have an effective working relationship with.

"I'm going to pick someone who is simpatico with me philosophically," he said in August.

Here are the main contenders to be Biden's running mate:

Kamala Harris (55)

Senator for California and contender for the 2020 Democratic nomination.

Strengths: The California senator has longstanding links with Biden, having been friendly with his son Beau when they were both state attorney generals. She also has national experience, is a strong media performer and would balance the ticket in terms of ethnicity – her mother emigrated from India, her father from Jamaica – and to a degree ideology – her Senate voting record is liberal, but not to the extent that scares moderates.

Weaknesses: Harris ticks most of the boxes, but she also started out her presidential run with a strong CV and didn't catch on with voters. Her state of California is solidly Democratic, and while Biden appears to have put her attack on him in the first Democratic debate behind him, his wife Jill described it as a "punch to the gut" last month.

Amy Klobuchar (59)

Senator for Minnesota and contender for the 2020 Democratic nomination.

Strengths: Klobuchar has national-level experience, and – as she reminded viewers repeatedly during her campaign for the nomination – has never lost an election in her home state of Minnesota, key in 2020, and could be an important player in other Midwestern battlegrounds. She also shares Biden's moderate politics, passes the "simpatico" test, and did the former VP a huge favor by pulling out of the Democratic race before Super Tuesday.

Weaknesses: Being ideologically similar to Biden may be appealing in terms of effective governing, but her record is unlikely to excite Sanders supporters and picking her would risk escalating the rift between centrist and progressive wings of the party. She has also faced allegations of mistreating staff and some problematic cases from her time as district attorney.

Stacy Abrams (46)

Almost won the Georgia governorship in 2018 and has lobbied hard to be Biden's VP.

Strengths: Abrams would balance the ticket in terms of age and race. She is a darling of Democrats with a powerful backstory, who has the potential to excite voters on all sides of the party. She has also made clear she wants the job.

Weaknesses: Experience. For all Abrams' strengths and media profile, the highest office she has held is House minority leader in Georgia. If Biden wants to pick someone who voters are confident can take his place in the Oval Office, Abrams is unlikely to be on the ticket.

Gretchen Whitmer (48)

Governor of Michigan, at the forefront of the COVID-19 response.

Strengths: Whitmer is the high profile governor of a key swing state. Long viewed as a figure with potential to go further, she gave the Democratic response to Trump's state of the union this year and has repeatedly locked horns with the president over their respective responses to the COVID-19 pandemic. Trump's dismissal of her as "that woman from Michigan" was a rallying call for supporters of the governor around the country.

Weaknesses: She is under intense scrutiny as governor during the pandemic – it has raised her national profile, but also carries risks. Embarking on a vice presidential run wouldn't look great at a time of crisis for her state. And while it's possible Michigan is virus-free by November, it's also possible the situation worsens.

Elizabeth Warren (70)

Senator for Massachusetts and contender for the 2020 Democratic nomination.

Strengths: Warren was an effective fundraiser and the standout debater – destroying Michael Bloomberg's candidacy – during the Democratic nomination process, but struggled at the polls. She is experienced, a household name and her liberal credentials would appeal to progressives. She could be a unity candidate, though risks turning off independents. 

Weaknesses: Despite a strong endorsement of Biden, they haven't always had the best of relationships. Age is also a potential problem – she's seven years younger than Biden, but still a septuagenarian. Her Senate seat in Massachusetts would be filled by an appointee from a Republican governor, off-putting in a year the Democrats hope to win back the Senate. 

Tammy Duckworth (52)

Democratic senator for Illinois and former army lieutenant colonel. 

Strengths: Duckworth is a Thai-American senator and former U.S. Army lieutenant colonel. She lost both her legs and partial use of an arm when her helicopter was shot down during the Iraq War, and, aged 50, was the first sitting senator to give birth. She is also a moderate, could have appeal in the Midwest and would undoubtedly be an inspirational choice.

Weaknesses: She hasn't been strongly vetted on the national stage and has upset some Sanders supporters by seemingly dismissing progressive star Alexandria Ocasio-Cortez as "the future of the party in the Bronx."

Best of the rest

Tammy Baldwin: The first openly gay senator when elected by voters in the key swing state of Wisconsin in 2012 could appeal to both Midwestern and young voters. But, like Warren, her seat in the Senate could be judged to valuable to risk in a special election.

Keisha Lance Bottoms: The Atlanta mayor has been championed by Clyborn, the South Carolina representative whose endorsement of Biden is credited with resurrecting his campaign.

Catherine Cortez Masto: The first Latina to be elected to the United States Senate. The Nevada senator lacks a national profile, but would be a fresh face if Biden is looking to spring a surprise.

Val Demings: The Florida representative and former Orlando police chief is from a key swing state and has serious law and order credentials. She was also one of the House managers during the Trump impeachment trial.

Michelle Lujan Grisham: The only non-white female governor in the United States – she has executive experience in New Mexico and would give the ticket balance in terms of race and age.

Susan Rice: Obama's national security adviser and ambassador to the UN knows Biden well, and as an experienced African American woman would help balance the ticket while holding the credentials to step into the top job. However, her reputation was damaged by the Benghazi killings. 

Michelle Obama: The former first lady would be the dream ticket for many Democrats – Biden said he'd pick her "in a heartbeat" – but she has repeatedly ruled out a run for political office.