On Saturday night, after Mr. de Blasio, a surrogate for Mr. Sanders, tweeted at Mr. Buttigieg, saying, “Dude, show some humility,” Mr. Lockhart immediately replied to the New York mayor, “Who cares what you think?”

Four or five years ago, Mr. Lockhart said in an interview, he would have had to phone a reporter to weigh in, and the mainstream press, acting as gatekeeper, might or might not have quoted him.

Now, “there’s the ability for people to have their own platforms,” he said. “I tweeted it, and got back to my dinner.”

David Pepper, chairman of the Ohio Democratic Party, lamented that the last debate, which smashed records as the most watched Democratic presidential debate ever, was so intensely negative. “My worry is that everyone looked so mad at each other,” he said. “When every person on the stage has their oppo ready to go and delivers it with such ferocity, you may win that individual skirmish, but you’re giving away the bigger picture — the need for people to be inspired.”

Yes, there are many examples of Democratic rivals turning negative in past primary races. Strategists for Hillary Clinton in the 2008 race dangled Mr. Obama’s youthful drug use. In 2016, Mrs. Clinton accused Mr. Sanders of smearing her for receiving donations from rich people.

But those digs feel innocent compared with this year, when a burst of gloves-off negativity seems to have been set off by the prospect of a staunchly leftist nominee in Mr. Sanders, the late entry of Michael R. Bloomberg and a fierce fight over which moderate candidate ought to be Mr. Sanders’s leading foil.

On Sunday night, Mr. Sanders praised Fidel Castro for Cuba’s literacy program on CBS’s “60 Minutes.” Many responses were searing from fellow Democrats, including two congresswomen from Florida with sizable Cuban electorates. “As the first South American immigrant member of Congress who proudly represents thousands of Cuban Americans, I find Senator Bernie Sanders’ comments on Castro’s Cuba absolutely unacceptable,” Representative Debbie Mucarsel-Powell, one of the Florida congresswomen, said on Twitter.