Fellow Democrats from New York say that the mayor must find some way to use the CNN town hall to give voters a reason to donate to his campaign, and boost his poll numbers ever so slightly.

“It’s going to be a test of whether or not he can say something that is so newsworthy that it can give him the same bump that a good debate would, since it doesn’t look like he’ll make the next debate stage,” said the Rev. Al Sharpton, a former presidential candidate and host of a show on MSNBC.

“I would never call one thing a make-or-break moment,” Mr. Sharpton added. “But I can’t see how, even after this, he has an easy path forward.”

If Mr. de Blasio was in need of a role model, he could refer to March 10, the date of Mayor Pete Buttigieg’s CNN town hall. Mr. Buttigieg, the mayor of South Bend, Ind., was a relative unknown before his CNN appearance; afterward, he received hundreds of thousands of donors and a deluge of attention.

“It was the singular most game-changing moment on the campaign,” said Lis Smith, a spokeswoman for the Buttigieg campaign who formerly worked for Mr. de Blasio’s first mayoral campaign. “Overnight, it launched him from being an unknown quantity to being in the hunt with U.S. senators and a former vice president in the polls.”

Ms. Smith’s free advice: Prepare, but don’t overprepare. You don’t want too many answers that seem scripted. Be clear about what you are bringing to the table that the Democratic Party and the country need.