Predictably, he was asked about the controversy over his medical records. Mr. Sanders is 78 years old and suffered a heart attack last fall. Afterward, he promised to make his medical records public. On Tuesday, he told CNN that his campaign wouldn’t be sharing anything beyond the three letters from doctors that it had released earlier. “I’m comfortable on what we have done,” he said. His campaign then set about attacking those who voiced concerns. His national spokeswoman likened questions to a “smear campaign,” before falsely claiming that Mr. Bloomberg, who had two stents implanted back in 2000, had also had a heart attack. (She later said she “misspoke.”)

For those not so “comfortable” with Mr. Sanders’s Trumpian lack of transparency, the debate offered little reassurance. When pressed on the issue, the candidate grew ever more flustered. He wound up in an embarrassing back-and-forth with Mr. Bloomberg over each other’s stents — just in case anyone watching had forgotten that both men are pushing 80. And he took to citing his cardiologists’ verdict that “Bernie Sanders is more than able to deal with the stress and the vigor of being president of the United States” — an echo of the 2015 letter from Mr. Trump’s doctor, stating that the then-candidate would be “the healthiest individual ever elected to the presidency.”

Worse still was Mr. Sanders’s response to questions about the divisiveness of his campaign, most particularly the vitriolic slice of supporters known as Bernie Bros. An aggressive subset of these fans, known for harassing those who criticize their man, were a problem in 2016, and they are a problem today. In Las Vegas on Wednesday, Pete Buttigieg pointed out that some of the senator’s acolytes were currently tangled up in a nasty fight with a powerful local labor union, the Culinary Workers, that had criticized Mr. Sanders’s Medicare for All plan.

The candidate’s response was, first, to play down the problem. “If there are a few people who make ugly remarks,” he said, “I disown those people.” He then sought to turn the tables, lamenting the “vicious, racist, sexist attacks” that the African-American women on his campaign had endured. He then suggested that it wasn’t his real supporters behaving badly, but maybe Russian bots. “I’m not saying that’s happening, but it would not shock me.” Or as a certain president might put it, “A lot of people are saying. …”

Mr. Buttigieg turned the screws. “We’re in this toxic political environment. Leadership isn’t just about policy,” he said. “Leadership is also about how you motivate people to treat other people. I think you have to accept some responsibility and ask yourself what it is about your campaign in particular that seems to be motivating this behavior more than others.”