New York City Mayor Bill de Blasio on Wednesday morning urged developers of below-market-rate housing to push state leaders to resuscitate the expired tax break known as 421-a — a key component of his promise to build thousands of such homes for the poor and middle class.

Addressing a midtown meeting of the New York State Association for Affordable Housing, de Blasio said he hoped builders, financiers and others in the audience could help persuade the state to bring back 421-a, which died in January, during the waning weeks of Albany’s current legislative session. It expired last year, but the state extended it for six months.

“We need support in the state capital. We need help,” de Blasio said on the podium of a ballroom in Times Square’s Marriott Marquis hotel. He added: “We’ve got five or six weeks left in the Albany legislative session. And it’s a chance for Albany to get it right now.”

Reviving 421-a is one of de Blasio’s Albany priorities, in addition to long-term renewal of mayoral control of public schools and other items that Gov. Andrew M. Cuomo, a fellow Democrat with whom he is openly feuding, and the Republican State Senate leader, John J. Flanagan of East Northport, whose colleagues de Blasio tried unsuccessfully in 2014 to unseat, must green-light. De Blasio’s ally in Albany is Assembly Speaker Carl Heastie (D-Bronx), but he is only one man of the proverbial three who control the state.

None of the three men’s aides returned calls or messages for comment Wednesday.

On 421-a, Cuomo has sided with construction labor unions, which want any future such projects to pay a union wage. However, developers and de Blasio said that would make such development too pricey to bring about the large-scale building needed to hit the mayor’s targets. De Blasio wants 421-a to be tweaked to allow for more affordable housing.

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De Blasio said on Wednesday he hoped the hundreds inside the hotel ballroom would help on the tax abatement, which city officials said was expected to work in tandem with the housing plan the City Council passed earlier this year.

“We all have to use all the impact we have in Albany. We have to reach our legislators. We have to let them know how important it is, and we have to put it in human terms.”

Without the break, the city would almost certainly need to spend more in order to allow the mayor to keep his word.

Said de Blasio: “Now, I am a believer that when something makes this much sense it actually can get done — even in Albany, New York.”