“He will be up there all alone,” said Melissa Mark-Viverito, a former speaker of the New York City Council and current candidate for a House seat in the Bronx, who often criticized Mr. Bloomberg’s City Hall as a progressive Democratic councilwoman. “And he has a lot of explaining to do.”
Mr. Bloomberg’s team is plainly mindful of the risks. Beyond the candidate’s own potential shortcomings off the cuff, he presents as a near-perfect foil for several others in the race. Mr. Sanders and Senator Elizabeth Warren have in recent days cast Mr. Bloomberg as the imperious emblem of runaway plutocracy. Pete Buttigieg, the former mayor of South Bend, Ind., and Senator Amy Klobuchar are expected to talk up their status as Midwestern underdogs, taking on a come-lately Democrat from the big city.
Former Vice President Joseph R. Biden Jr., the fading former front-runner whose stumbles helped persuade Mr. Bloomberg to run in the first place, has been pressing Mr. Bloomberg on his longstanding support for stop-and-frisk policing, an attack that Mr. Biden hopes will resonate with the black voters who remain at the core of his support base. Mr. Biden has watched Mr. Bloomberg stuff the airwaves with ads implying a chummy partnership with former President Barack Obama, with whom Mr. Bloomberg was never particularly close.
“If I had that money, I guess I’d run my ads, too,” Mr. Biden said in New Hampshire last week. “Look, let’s get into the debates, OK? Let’s get into the debates. We got a lot to talk about.”
The last several days have only added to the prospective discussion areas, aided by a steady drip of dredged-up clips of Mr. Bloomberg speaking unguardedly before he began running. On Tuesday, BuzzFeed News reported on 2019 footage of Mr. Bloomberg referring to transgender people as “he, she or it,” the same day the campaign promoted a video highlighting Mr. Bloomberg’s commitment to “LGBTQ+ youth.”
Until recently, the Bloomberg campaign had been content to run in a kind of parallel primary, rarely engaging with fellow Democrats, effectively treating the February slate as a kind of qualifying tournament for the rest of the field, its winner empowered to take on Mr. Bloomberg in the main bracket. Mr. Bloomberg is not competing in the first four nominating contests, turning instead to the delegate-rich states that vote in March or later.
But after the Democratic National Committee changed its rules to facilitate Mr. Bloomberg’s inclusion — past debates have required candidates to meet certain individual donor benchmarks, and Mr. Bloomberg is not raising money from others — aides recognized that he had little choice but to participate if he qualified.