Could it be that the fate of New York City is being decided right now? Mayor Bill de Blasio thinks so.

At a rally on Wednesday, he told retirees and union members: “With your help, people are going to look back on these days in March of 2016, and they’re going to say that’s when an affordable city was saved. That’s when we stopped this city from going over the brink.”

The brink he means is the gaping divide between rents and incomes that many fear is splitting the city into two encampments: the rich and the homeless. Mr. de Blasio’s solution — a 10-year plan to build or preserve 200,000 affordable apartments — is facing a critical test this month. The City Council is nearing a vote on a proposal to rezone nearly 200 blocks of East New York, Brooklyn, allowing developers to put up taller buildings there on the condition that they also build thousands of new affordable apartments. It would be the first of 15 rezonings citywide that the administration hopes will increase the housing supply enough to ease the pressure on rents.

This grand plan is a hugely important gamble that the city can build its way to greater housing equality, that it can ignite growth, improve neighborhoods, tame gentrification and protect tenants, all at once. That New York can still be a mixed-income city.