As I've been saying for months, here is the key to Harris: look at South Carolina, look at black votes in South Carolina, look at black votes in other southern states. Up to the debate, all of those numbers have been SOLID for Biden: anywhere from 45-55% and nobody else close. If that stays roughly the same, Biden will be the nominee no matter what else happens. If those numbers for Biden start to go down, and Harris starts to rise, she's got a real shot.
Keep in mind: Harris does not need to win South Carolina. But she needs to prevent Biden from winning there decisively. She needs to at least come close.