Karen Bass, a Democratic congresswoman from California, has emerged in recent days as among the most tipped of the half-dozen contenders from which Mr Biden will announce his choice.
But Ms Bass, 66, is fighting off controversy after a right-wing website unearthed video of her from 2010 attending a ribbon-cutting ceremony for a new Church of Scientology building in Los Angeles, when she served on the state legislature.
In the video, published by the Daily Caller, Ms Bass calls for all humans to be treated equally. But she also praises the controversial group and its founder, L Ron Hubbard, who died in 1986.
“The Church of Scientology, I know, has made a difference, because your creed is a universal creed and one that speaks to all people everywhere,” she says in the video.
“That is why the words are exciting of your founder L Ron Hubbard, in the creed of the Church of Scientology: That all people of whatever race, colour or creed are created with equal rights.”
As The Hill pointed out, in recent years the church has faced accusations of widespread sexual abuse within the organisation, allegations it has denied. In 2018, the organisation settled a lawsuit with a woman who alleged that the she was forced by the church to have an abortion and then prevented from leaving.
In a statement posted on Twitter, Ms Bass said she had been trying to find an “area of agreement” with the church.
“Back in 2010, I attended the event knowing I was going to address a group of people with beliefs very different than my own, and spoke briefly about things I think most of us agree with, and on those things — respect for different views, equality, and fighting oppression — my views have not changed,” she tweeted.
“Since then, published first-hand accounts in books, interviews and documentaries have exposed this group.”
This is not the only controversy Ms Bass faces, as she seeks to become the first African American woman selected by a major party as its vice presidential running mate.
Last week, The Atlantic reported that Ms Bass worked in Cuba in the 1970s with a group aligned with Fidel Castro’s government.
“She was always pro-Castro & later mourned his death,” tweeted Trump campaign spokesman Tim Murtaugh. “Whether Biden picks her or not, he’s written off Cuban-American voters just by considering her.”
Speaking to MSNBC last week, Ms Bass, a five-term legislator, said she had spoken to her colleagues about comments she made at the time about the Cuban regime.
“I have talked to my colleagues in the House about that, and it’s certainly something that I would not say again,” she said. “I have always supported the Cuban people, and the relationship that Barack Obama and Biden had in their administration in terms of opening up relations.”