Mayor de Blasio set up towering expectations that his vision to build or preserve 200,000 affordable apartments would end the "tale of two cities" he condemned on his first campaign trail.

A crowd of the very voters he appealed to — mostly black and Latino, with modest incomes, deep city roots and big dreams — will rally by the thousands at City Hall on Monday to demand a new chapter, for the salvation of communities under siege by high rents, real estate speculators and crumbling public housing.

De Blasio ought to give those organized in conjunction with Metro-IAF, from congregations and community groups across the boroughs, his full and eager attention. Unfortunately, so far they've gotten something closer to the back of his hand.

The mayor is coasting toward reelection, fortunate to have no serious challenger. But any victory will be Pyrrhic if the very people he claims to champion continue to flee their neighborhoods and their city for want of the boldest action possible to reimagine solutions to the city's affordable housing crisis.

And let there be no doubt. For hundreds of thousands increasingly unable to afford their city, this is a full-fledged crisis.

Grant this much to de Blasio: His housing plan resourcefully squeezes affordable units out of new apartment buildings and meanwhile invests billions into erecting new housing for the needy, even with too little help from the federal government and too few vacant city-owned parcels left. Hearing loud and clear that rents at first weren't low enough, this year he added $1.9 billion more in capital funds.