Certainly, Mr. Bloomberg has never debated anyone quite like Mr. Sanders, whose allies consider Mr. Bloomberg’s very biography an affront to people-powered democracy. Mr. Wolfson has been playing Mr. Sanders in the mock forums, taking inspiration in part from the curmudgeonly Muppets characters Statler and Waldorf, he said. The impression has occasionally moved Mr. Bloomberg to fits of momentum-breaking laughter during their practices, according to an aide.

Asked if Mr. Bloomberg was rusty after so many years out of office, Mr. Wolfson did not say no. “We’ll find out Wednesday,” he said.

Even supporters would not describe Mr. Bloomberg as a rousing public speaker. His address endorsing Hillary Clinton at the 2016 Democratic National Convention, part of her effort to highlight support from moderates and disaffected Republicans, is best remembered for his minimalist plea to elect “a sane, competent person” as president.

He is prone more often to groaners than zingers. Alluding to Mr. Sanders once last month, Mr. Bloomberg called himself the only Jewish candidate “who doesn’t want to turn America into a kibbutz.”

And when challenged by journalists, Mr. Bloomberg can quickly become defensive. Shortly after he announced his bid, a television interview on “CBS This Morning” produced an arresting volume of notable remarks. Among them: the suggestion that other candidates “had a chance to go out and make a lot of money” to pour into their own campaigns, so they should not begrudge him for having done so; and the assertion that “nobody asked” about Mr. Bloomberg’s defense of stop-and-frisk policing until he apologized for the strategy in November. (Many, many people had asked — and sued, and protested — throughout his time as mayor, and he defended the approach as recently as last year.)

Veterans of Mr. Bloomberg’s City Hall runs say that for any missteps he might have had in front of the cameras, few came during debates.

“He wasn’t really ever trying to knock anybody out,” said Edward Skyler, a 2001 campaign aide who became one of Mr. Bloomberg’s deputy mayors. “He could absorb a hit and move on. Somebody would go on the attack and he’d just be like, ‘OK.’”