Suppose, to take one scenario, the president declares himself the victor before the victor is known: “What a landslide, this is fantastic, the polls and pundits were wrong!” Maybe it will be his supporters or family members who declare victory.

And what if in the following days and weeks the count changes? What if on day five, or 10, or 30, Mr. Biden looks like the winner?

That’s the general area when things could go very wrong. “Postelection through to the inauguration, we have a real danger zone,” says Larry Sabato, the great veteran director of the University of Virginia’s Center for Politics.

There will be charges and countercharges, rumors, legal challenges. There will be stories—“My cousin saw with her own eyes bags of votes being thrown in the Ohio River.” Most dangerously there will be conspiracy theories, fed by a frenzied internet.

Let’s make the picture darker, to deepen the point. What many people will fear in such an atmosphere is the possibility of violence. We’ve just been through a round of street violence this summer. It is not beyond imagining that in a tense national environment we would see it again. Maybe it will be Black Lives Matter and antifa versus white nationalists and QAnon. Maybe it will include honestly enraged citizens who believe their side was wronged.