I think it was the moment when polls came out showing Sanders ahead of Biden, with Warren still in a tie for third with everyone else and Harris out of the race. That's the moment we can point to and say "this is when Hillary Clinton realized that she'd never be able to play Everyone's Second Choice and use that to become President."

Whoever came up with "Bernie Bros" will have a job in political campaigning for the rest of their life, because that was amazingly effective.