“Other than Harris going after Biden and Gabbard going after Harris, they have been very timid,” said Bob Kerrey, the former senator and Nebraska governor who ran for the Democratic presidential nomination in 1992. Part of that, he said, is the impossibility of a deeply substantive debate among 10 people, which is how many were onstage for each of the June, July and September debates. The coming debate will have 12 people onstage.

And timidity won’t do. “Trump will be well funded and ferocious,” Kerrey told me. “If a Democratic candidate wilts under the warm attention and criticism of another Democrat, they will not be able to stand up to Trump.” And if that criticism goes too far, bleeding into gratuitous nastiness, “the audience will discipline them,” he said. He mentioned Julián Castro’s obvious insinuation in the debate last month that Biden was in serious cognitive decline.

That went too far. It was counterproductive, because it didn’t bring to light something about a candidate that wasn’t accessible to voters. That was true as well of Eric Swalwell’s “pass the torch” hectoring of Biden, in a June debate, as a politician who was overstaying his welcome. Voters know that Biden is 76 and has spent more than four decades of his life in public office. Swalwell wasn’t exposing some secret that had been swept under the rug; he was just stamping his feet.

Many anxious, Trump-horrified Democrats take the view that the candidates should tread lightly on one another. “When we know that President Trump will be spending his money to discourage Democratic turnout more explicitly and underhandedly than in any other modern campaign, why would we want any candidate to spend time persuading Democrats of how bad other Democrats are?” asked Jack Markell, the former governor of Delaware. “The difference between all of the Democratic candidates is dwarfed by the difference between the Democratic candidates collectively and President Trump.”

Many of the readers who write to me agree with him or go even further: They want to see, hear and revel in nothing but loving encomiums about Democrats vying for the White House. Some of them believe that Hillary Clinton was badly hurt by her battle with Sanders in the last Democratic primary, and that we’re where we are as a result.

But her wounds would have mattered less if Sanders had rallied more enthusiastically to her side once she’d won the nomination. I urge the losers this time around to get in line pronto. And I think it’s extremely risky to spare the next nominee the sort of tough examination that will let us know if we’ve elevated the most broadly appealing, durable, fearsome Trump slayer.

The stakes could hardly be higher. Every day, new revelations about the Ukraine sordidness come to light. Every day, the president acts in impulsive, imprudent, unhinged ways. Maybe that means that any Democrat can beat him. But I’m a bit of a pessimist and cautious to a fault. I think it means that we cannot afford to pick the wrong person.