Putting aside his warm relations with donors in the credit card industry, Mr. Biden’s record is mottled with more episodes of apparent corruption than Ms. Teachout listed in her op-ed. In a 2008 episode of “Meet the Press,” Tom Brokaw interrogated Mr. Biden about the fact that his son, Hunter Biden, was employed by MBNA, a bank holding firm — which also happened to be a major donor to the elder Biden, and a strong advocate for the bankruptcy bill he worked to pass while his son cashed MBNA checks.

It is hard to imagine how the younger Biden would have become “the very model of a modern major lobbyist,” as Politico put it in a 2007 article, “with an office near K Street, a blue-ribbon roster of clients,” and a firm grossing $1.76 million in lobbying revenue in the first half of 2006 alone were it not for his parentage. A transcript of the hearing on Hunter Biden’s nomination to Amtrak’s board of directors shows that while the other nominee joining him that day, Donna McLean, had a 15-year career in federal service with a focus on transportation policy, Hunter’s specific qualification for the position was that he often rode trains. Hunter was introduced by Senator Tom Carper of Delaware that day not principally as a logical choice for Amtrak’s board, but as the son of Joe Biden.

The fact that the Bidens are entangled in Trump’s impeachment odyssey — uncomfortably, and with no clear exit strategy aside from keeping mum — should also raise serious questions about Mr. Biden’s fortitude in the general election. But all this, Mr. Sanders declined to say — not because it isn’t relevant and not because it doesn’t fit with Mr. Sanders’s platform — but because Mr. Sanders, for all his famed gruffness, doesn’t like going negative. “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. The financial capture of Washington that he rightfully detests is real, and it is manifest in his opponents. He will have to own that, or risk campaigning hobbled. There isn’t any way around it; the logic that demands it is inscribed into every speech Mr. Sanders makes.