It was, on its face, a straightforward question: What were the nine mayoral candidates’ biggest political regrets?

Yet the question elicited one of the more revealing and contentious exchanges on Wednesday night in the first debate in the Democratic primary race for mayor in New York City.

Zohran Mamdani, the 33-year-old state assemblyman and Democratic Socialists of America member consistently polling in second place, suggested there was a time that he trusted the Democratic Party leaders “like Andrew Cuomo,” and that he now considered that a mistake.

Mr. Cuomo, the current front-runner, whose decades in politics come with both accomplishments and baggage, struggled to summon a single regret from memory. But he did make a point of lamenting Mr. Mamdani’s rise.

All of which prompted Adrienne Adams, the City Council speaker, to interject, her voice laden with incredulity.

“No regrets when it comes to slow-walking P.P.E. and vaccinations in the season of Covid to Black and brown communities?” she asked, shaking her head with disapproval. “Really, no regrets? No regrets?”