By contrast, Mr. Trump will absolutely aim to get under Mr. Biden’s skin in hopes of provoking a game-changing gaffe. Say what you will about the president, no one doubts his ability to get under people’s skins.

This poses a particular danger for Mr. Biden. Over the course of this campaign, he’s already left a trail of gaffes. Biden supporters say they don’t mean much because they’re already baked into the public perception of the man.

Well, yes and no. It’s one thing to garble a word (on Sunday he said “N1H1” instead of H1N1), or to exhort a man in a wheelchair to “stand up” at a campaign rally, as then-Sen. Biden did in September 2008. Even people who laugh can well imagine themselves making the same mistake. It’s another thing to do what he’s done this past year—saying he’s in Vermont when he’s in New Hampshire, making up tales such as getting arrested in South Africa while trying to visit Nelson Mandela, or recalling how he’d met as vice president with students who’d survived the school shooting in Parkland, Fla.—which didn’t take place until 2018. These are the things that have people questioning whether he’s all there.

Mr. Biden’s particular weakness has to do with his anger.