From a Jack Shafer Fourth Estate column on politico.com headlined “Why Democrats Should Primary Biden”:

In the middle of the last century, prizefighters who weren’t fighting frequently enough to keep sharp would schedule tuneup matches with capable boxers to shake the ring rust out of their form. Their managers wouldn’t pick palookas or chumps but boxers who could challenge their guy in a way that would reveal his weaknesses and indicate what part of his game needed more training.