We know for sure that the former vice president will pick a woman, and he is believed to be considering several women of color.
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He said he would make his decision this week, although his campaign has raised the possibility that a formal announcement wouldn't come until next week, according to the Associated Press.
Only an extremely small circle truly knows Biden's thinking regarding his running mate, and those people aren't talking.
Here are a few likely picks, in alphabetical order:
Karen Bass, U.S. representative from California (2011-present)
What to know about Rep. Karen Bass
Karen Bass is a California representative with a reputation in Washington as a bridge-builder. She joined Congress in 2011 and now chairs the Congressional Black Caucus.
Bass previously served as House speaker in California's legislature, where she helped mend partisan tensions amid crippling budget cuts during the 2008 Great Recession.
A former physician assistant and community organizer, Bass said she brings experience to tackle the nation's economic, racial and health care crises.
Both Biden and Bass learned to heal through personal tragedy. Biden lost his wife and young daughter in a car accident in 1972. In 2006, Bass's 23-year-old daughter and son-in-law died in a car accident.
Yet her low-key approach to politics means she is lesser-known to voters and lacks a major donor base.
She's also faced pushback for calling the 2016 death of Fidel Castro a "great loss to the people of Cuba." Bass recently said she was unaware of the phrase's political significance in Florida at the time.
Tammy Duckworth, U.S. senator from Illinois (2017-present)
What to know about Sen. Tammy Duckworth
Tammy Duckworth is an Illinois senator and trailblazer on Capitol Hill, becoming the first woman with a disability elected to Congress, the first Thai-American senator and the first senator to give birth in office.
She's the only top VP contender who's also a combat veteran. She was deployed to Iraq in 2004 as a Black Hawk pilot and was among the first women in the Army to fly combat missions. Duckworth, a Purple Heart recipient, lost both of her legs and partial use of one arm when her helicopter was hit by a rocket-propelled grenade.
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She previously served two terms in the House and as the Assistant Secretary for Public and Intergovernmental Affairs in the Department of Veteran Affairs under the Obama administration.
On the flip side, Duckworth has made headlines throughout her career but hasn't been a consistent force yet in Washington.
Kamala Harris, U.S. senator from California (2017-present)
What to know about Sen. Kamala Harris
Kamala Harris is a California senator and former candidate in the 2020 presidential election.
Harris has an impressive resume in politics, was vetted on the presidential stage and has been elected statewide multiple times.
Born the daughter of Jamaican and Indian immigrants, she served two terms as San Francisco's district attorney and later as California's attorney general, the first woman of color to hold that office.
Harris is actively fundraising for Biden since she endorsed him earlier this year and appeared in several online campaign events during the COVID-19 pandemic.
Her time on the debate stage showed that she can easily take down opponents, but that includes Biden himself.
In 2019, Harris slammed Biden's opposition to busing as schools began to integrate. "There was a little girl in California who was a part of the second class to integrate her public schools ... and that little girl was me," she said on the debate stage.
Also, in the aftermath of George Floyd's killing, Harris' prosecutor record, especially on drug offenses, was called to question.
Susan Rice, former ambassador to the United Nations
What to know about Susan Rice
Susan Rice served in the Obama administration as ambassador to the United Nations and National Security Advisor.
Her long resume in foreign affairs includes a stint in the Clinton administration as assistant secretary of state for African Affairs.
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Rice is close to Obama and his network of political advisers and has a personal relationship with Biden, which is seen as one of her strongest advantages.
Though Rice has an extensive career in government, she has never run for or held public office. Rice was on track to become Obama's second-term secretary of state but became ensnared in the political controversy over the administration's handling of the 2012 attack on Americans in Benghazi, Libya.
Elizabeth Warren, U.S. senator from Massachusetts (2013-present)
What to know about Sen. Elizabeth Warren
Elizabeth Warren is a Massachusetts senator and former 2020 presidential candidate known for her progressive politics.
Born in Oklahoma, she attended the University of Houston and Rutgers School of Law and initially worked in academia.
She emerged as a national figure in the aftermath of the 2008 financial crisis when she chaired the Congressional Oversight Panel, a relevant experience considering the pandemic's economic fallout.
She then worked under the Obama administration to establish the Consumer Financial Protection Bureau before winning her Senate seat in 2012.
After dropping out of the 2020 campaign and waiting to endorse Biden, Warren has emerged as a policy advisor to his campaign.
Though Biden's embraced some of Warren's positions, a Biden-Warren ticket could be used to cast Biden as a "tool of the radical left" even if she's not so radical herself.
Gretchen Whitmer, governor of Michigan
Whitmer met with Biden as he nears VP decision: Sources
Gretchen Whitmer is a first-term governor of Michigan, a crucial battleground in 2020 that President Trump won by the narrowest of margins.
She previously served in the Michigan state legislature for 14 years and in the state Senate for nine until she was term-limited.
Biden and Whitmer formed a bond after he campaigned for her in the 2018 gubernatorial election. She is a co-chair on his campaign.
Her profile has grown since delivering the Democrats' response to Trump's State of the Union address and especially amid the pandemic. She has taken aggressive steps to curb the coronavirus in a state that was a hot spot nationally early on and -- after she criticized the federal response -- has drawn criticism from Trump.
Yet Whitmer has only held executive office for less than two years and no federal experience, leaving questions about whether she is prepared to step into the role of vice president.
Someone else?
It's always possible the choice will be someone else. Other contenders on his shortlist include voting rights activist Stacey Abrams, Atlanta Mayor Keisha Lance Bottoms and Florida Representative Val Demings.
The Associated Press contributed to this report.