As the Democratic presidential candidates bandage their wounds and assess their strategies after Wednesday’s debate, one thing is clear: Bernie Sanders owes Mike Bloomberg a big thank-you bouquet. Maybe even a box of chocolates.

Sure, the multibillionaire former mayor, in his debate debut, took a swing or two at Mr. Sanders, the race’s front-runner pro tem, repeatedly asserting that he had no chance of beating President Trump. But nothing said to or about Mr. Sanders mattered nearly so much as Mr. Bloomberg serving as the evening’s piñata, drawing blows away from the Vermont senator.

Without Mr. Bloomberg, Mr. Sanders not only would have suffered more hits, but more people would have focused on how clumsily he handled those he did draw.

In debates, Mr. Sanders has one mode: shouty. It fits with the chronic crabbiness his fans find so charming — evidence of his passion and authenticity. But when you combine shouty with defensive, the result is not so charming, which is where he found himself now and again on Wednesday night.

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.”

This is not to pick on Mr. Sanders alone. The Las Vegas event was far more combative than previous debates, providing a clearer look at how most of the field responds to sharp attacks — a useful bit of knowledge considering whom the eventual nominee will face in the general election. The nominee needs not only to be able to throw a punch, he or she needs to be able to take one.

Elizabeth Warren fielded attacks reasonably well. This may be in part because she got some hard-won experience back in the fall, when she was briefly the front-runner. She grew frustrated when Mr. Buttigieg insulted her Medicare for All plan, and she and Joe Biden sparred over the party’s handling of the Senate Republican leader, Mitch McConnell. But she didn’t respond much above her baseline demeanor — a mild-to-moderate exasperation — and she didn’t get nasty or lose her line of argument.

Amy Klobuchar spent much of the evening visibly miffed, especially when clashing with Mr. Buttigieg. Ms. Klobuchar is often at her best when fired up about an issue. But Mr. Buttigieg pushed her to lose her cool on several occasions. Most vividly: In a recent interview, Ms. Klobuchar could not come up with the name of the president of Mexico (Andrés Manuel López Obrador). Mr. Buttigieg kept tweaking her about that until finally she demanded: “Are you trying to say that I’m dumb? Are you mocking me here, Pete?”

Mr. Buttigieg, as is his way, kept calm, staying on message and deflecting personal attacks about his experience — or lack thereof. On occasion, he drifted over the line into smugness or condescension, dangerous ground when going up against Ms. Klobuchar. (Nobody likes a mansplainer.) And his unflappability will continue to infuriate those who find him too smooth and aloof, seeing it as proof that he does not feel their pain and outrage.

Mr. Biden didn’t take much of a beating — it feels as though no one has the heart to really go after him at this point — and he handled criticism in an unremarkable fashion.

Then of course there’s Mr. Bloomberg, who responded to the beat-down by turning peevish and evasive, stumbling through grudging non-apologies for past misbehavior on matters of both policy (stop-and-frisk) and character (his reputedly sexist and demeaning treatment of women who worked for him). He has the time (and money!) to recover, but he did real damage to his Trump-slayer narrative.

With yet another debate, in South Carolina, less than a week away, Mr. Sanders in particular would do well to up his game. A bloom-off-the-rose Mr. Bloomberg is unlikely to provide as much cover for the front-runner next time around.