Yet Mr. Trump’s own chaotic governance has too often handed his enemies a sword. His narcissism made him think he could control FBI director James Comey, and his indulgent tweet about taping Mr. Comey triggered a special counsel. He is often reckless and makes needless enemies. He is not the only cause of America’s political divisions, but he has contributed to them. He has had four chiefs of staff, four national security advisers, and he often trashes good people as they depart.

As we warned in 2017, Mr. Trump has also squandered his ability to persuade with false claims. That lack of credibility came home to haunt with Covid-19. Mr. Trump’s policies—on vaccines and resources—are better than his critics claim. But his habit of personalizing everything, engaging in petty feuds, and making Panglossian claims of “rounding the turn” have cost him support, especially among seniors.

There’s no reason to think Mr. Trump’s governance would change in a second term. His disruption worked in 2016 against Hillary Clinton, but he has returned to the same playbook this year when the public is in a different frame of mind. Americans want calm realism on Covid. For all of his cunning and marketing flair, he has missed this change in the national mood. By focusing so much on himself, he has helped Mr. Biden make the election a referendum on the incumbent rather than a choice.

The best argument for re-electing Mr. Trump is as a brake on a Democratic Party that is increasingly radical on the economy and the culture; Mr. Biden won’t be able or willing to do it. A GOP Senate could also be that brake, though the Senate’s fate may depend on how well Mr. Trump does. Covid and partisan passions make this election even harder than usual to handicap. Voters who want a centrist President Biden should split their tickets.