The custom is to review debates—and especially the “viral moments” they sometimes produce—on stylistic grounds, on the theory that in the modern presidency style and substance are intricately intertwined, and a high-profile encounter between rivals with tens of millions of people watching provides a good window into leadership character. In normal times that theory probably has a decent measure of truth.

But how many viewers Thursday night, or catching up on the proceeding Friday morning, could be so credulous that they somehow found their 2020 choice illuminated in a vivid new way?

Never before has the central question in a presidential race been framed in such a towering way: Do you approve of the incumbent’s disruptive approach to the presidency, and the world, or do you regard him as a menace to important institutions, customs, and values?