For Mr. Mamdani to be effective, he will need to grapple with the recent history of big-city civic leaders promising bold, progressive change. They have mostly delivered disappointment, including in Chicago, San Francisco, Seattle and Portland, Ore., as well as in New York City itself under Bill de Blasio. They have tolerated a level of disorder that many voters found unacceptable and struggled to make progress on affordability or K-12 schools. Mr. de Blasio had one major success that Mr. Mamdani justly cites as an inspiration: universal pre-K, which has helped hundreds of thousands of children. Still, Mr. de Blasio’s overall record made him so unpopular that he could not win the Democratic nomination for a House district in Brooklyn after he left office.