Biden’s biggest advantage in the post-Labor Day sprint is that he has an even easier presidential act to follow than Franklin D. Roosevelt had running against Herbert Hoover in 1932. But this advantage could evaporate if rioting and looting continue, and millions of voters become convinced that Democrats are complicit in — because tolerant of — the shredding of the nation’s social fabric. Is or is not Biden disgusted by mob violence in the service of political nihilism? (“Let’s protest police injustices by torching an automobile dealership!”)

He needs a Sister Souljah moment. In 1992 this rap singer was pleased by the deadly Los Angeles riots following the acquittal of the police officers involved in the Rodney King beating: “If black people kill black people every day, why not have a week and kill white people?” Candidate Bill Clinton’s criticism, not of extremism in general but of her explicitly, reassured temperate voters that he was not intimidated by inhabitants of the wilder shores of American politics.

Today, even more than 28 years ago, the Democratic nominee needs to display similar independence. Biden’s response last week — 43 seconds of a tepid, 93-second video tweet — will not suffice.