What comes to mind for me:

"It's a big club and you ain't in it." - George Carlin

AKA, my impression is: A guy like Biden is willing to do this because on some level, he knows the differences between him and Trump aren't actually that big, and that the divide between the two ruling parties is more performance art than visceral disagreement. And he's not supposed to let this show so blatantly, but he's slipping in his aging state of mind.