Joe Biden’s first year as president is almost over, and people are already trying to find a replacement for him. To dampen speculation, Biden said earlier this week that he was primed for another run in 2024 and would “relish” the challenge of going against Donald Trump in a rematch of 2020. But that’s boilerplate. It’s what the president is supposed to say to head off a brutal internecine fight between the radicals and the nearly radical. How much longer he can hang on to the illusion that he will run in 2024 is hard to say. For the moment,...