For many months as Biden considered nearly a dozen women for the ticket, California Sen. Kamala Harris seemed like the most natural fit because of her resume, her visibility supporting the Black Lives Matter protests, and her championing of legislation to curb police brutality. But over the past week, long-held uneasiness among some close Biden allies about Harris' ambition and her attack on Biden during a June 2019 debate broke into the open.

Then this weekend, it was California Rep. Karen Bass' turn in the hot seat, as the Trump campaign highlighted her trips to Cuba as a young activist and questioned whether Biden would "put Castro-loving Communist Karen Bass a heartbeat away from the presidency" -- underscoring the potential risks for Biden in choosing a politician who is largely unknown as the other side races to define them.

View Trump and Biden head-to-head polling

Though vice presidential picks have rarely had a major effect on the outcome of an election, Biden's pick has taken on an outsized importance this year -- not only because of voters' concerns about his age, but also because in the midst of a pandemic and a painful recession, his team knows he must heed the first rule of vice presidential politics by picking a loyal partner who does no harm.

The spectacle of the vice presidential vetting process breaking into public view illustrated the difficulty of the decision facing Biden as he narrows his choice, as well as the behind-the-scenes maneuvering by the potential hopefuls and those who would vie to replace them in their current roles. (The current parlor game in California political circles, for example, is guessing who Democratic Gov. Gavin Newsom might appoint to an open US Senate seat if Harris vacates hers.)

Though Biden's team has kept the process secret, the former vice president made it clear from the beginning that he would pick a woman, and his team has vetted Bass, Harris and former Obama national security adviser Susan Rice -- who are widely viewed as top contenders -- as well as Massachusetts Sen. Elizabeth Warren, Illinois Sen. Tammy Duckworth and Florida Rep. Val Demings. Others under consideration include Atlanta Mayor Keisha Lance Bottoms and Michigan Gov. Gretchen Whitmer.

'There will be a resistance to your ambition'

For many months, it was no secret that some of Biden's allies were still galled by Harris' attack on Biden during the 2019 debate in Miami, where she called out Biden's opposition to busing decades earlier as a young Delaware senator and his boasts that he had found a way to work with segregationist senators (comments she said she found hurtful).

Her knockout punch in Miami gave her a fleeting boost in the polls, but became a potential strike against her. Biden looked stunned and hurt in that moment, in part because the two Democrats had been longtime allies and because Harris had been close with his late son Beau Biden, when they had served as attorneys general at the same time, she from California and he from Delaw