You think he REALLY doesn't see Biden as the bigger threat? That Biden doesn't appeal to a certain amount of his base? That his internal polling and analytics have told him who he has the best and worse chances against? Trump also knows that, weirdly enough, he and Bernie have a weird cross-section of support and he's speaking directly to those folks. I think the evidence has shown that he will do anything he can to damage Biden, who he sees as the bigger threat. I mean, he's getting impeached because of Biden for god's sake.