“I’ve never run a negative ad in my life,” Mr. Sanders told reporters upon announcing his 2016 primary bid, “I hate and detest these ugly, 30-second negative ads.” True to form, the single attack ad Mr. Sanders ran during his race against Hillary Clinton contained neither her name nor her visage, only a reference to Goldman Sachs’s financial hold on Washington.

Contrast that with the sort of material then-Senator Barack Obama aired against Mrs. Clinton in 2008 — claiming, for instance, that she would say anything to get elected, that she didn’t tell the truth, that she was a “corporate lawyer sitting on the board of Walmart ” — and Mr. Sanders’s forays into offense in 2020 appear quite mild.

That is because they are. But Mr. Sanders can’t afford mildness, or a steady habit of capitulation. The critical distinction between Bernie Sanders and the donor-driven, suborned, anti-democratic politics he hopes to overthrow is his greatest virtue, and his strongest appeal. But for that distinction to be more than an abstraction, Mr. Sanders will have to illuminate the contrast between the way he practices politics and the way his opponents do. This will mean running attack ads, devoting stump time to his opponents’ unsavory histories and standing behind surrogates who do the same.