Two observations:
One way Biden could lose is by getting infected.
He's an old guy and if he became CV sick or hospitalized, then people would be left possibly voting for a black woman (I think this was a danger for Biden no matter who he picked as Veep). Some mild symptoms and a quick recovery probably wouldn't hurt much, but would cause tension for a time. I was surprised how Bernie's heart attack didn't affect much, he returned, campaigned and that seemed to be that. Of course that was just the primaries and Dems are a little better behaved.
Changing gears, this is the second black person on a major ticket, and so far both have been mixed race, with both of their black ancestry related to immigrants and derived from outside the country, growing up without traditional black American (West African slave derived) relatives. Obama was raised by his white mother and her family. Though of course his skin color and hair (and our racist classification system) made him a black man in America. And he lived those experiences every day. But he also lived in white culture every day at home too. As an adult he did marry into a black family.
Harris has an Indian mother and Jamaican father. A rather different home experience than most black people in America. As with Obama, no family/blood connection to the mainstream black culture growing up. While Kamala Harris is probably considered black (or now Black) by most, she looks mixed race. And she married a Jewish lawyer.
Not to make too much of it, but I'd posit that Americans can handle race better when the person in question isn't too Black in presentation, outlook, skin tone. Mixed race, somebody with one foot outside the black community and able to speak to whites in their way, is more acceptable. I really wondered how say a Stacey Abrams would go over, a large dark-skinned black woman that probably scares many whites. One thing about race is there are plenty of gradations and America is coming around slowly. While there's some gender fluidity, it's more fixed with less intermediate/transition ground. And the gender intermediate zone is undoubtedly more scary to most Americans, while with race the middle ground is more comforting.
Hope this doesn't come across as insensitive, and I don't want to take away from Kamala Harris' moment or Obama's achievements. But I wonder when America will be ready for a dark-skinned POTUS with two Black parents.