The coronavirus pandemic is the greatest challenge Donald Trump has ever faced. As stock markets fall and patient numbers rise, the epidemic threatens the lives of some Americans and the prosperity of all—and it has already begun to disrupt the political methods that brought Mr. Trump to the White House.

As he has done in other crises, the president is stalling for time as he processes the nature of the threat and tests rhetorical and policy responses to it. But unlike human political adversaries, the coronavirus isn’t something...