Supporters of Joe Biden are unlikely to be persuaded by most of the common criticisms. They know he can be rambling and unintelligible. They know his record is unimpressive and that he doesn’t really have “policy proposals”. None of this matters, though, because to them he has the most important quality of all: he can beat Donald Trump. Nothing you can say about the former vice-president’s record, platform or mental state matters next to the argument that he is the best hope Democrats have of getting Trump out of office. There’s just one problem: it’s a myth. It is a...