District Council 37, the city's largest public union, will endorse City Comptroller John Liu in the mayoral race.
Campaign and union sources said Tuesday night that the 121,000-member union voted to back Liu, The Daily News reports.
The endorsement is a much-needed boost for Liu, who is lagging behind the other candidates in the polls.
In the latest Marist poll, City Council Speaker Christine Quinn leads with 24 percent, Anthony Weiner follows with 19 percent, Public Advocate Bill de Blasio has 12 percent, former Comptroller Bill Thompson has 11 percent and Liu trails with 8 percent.
Liu has close ties with DC 37, the News reports. But the choice further divides the city's labor movement.
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The healthcare workers of 1199SEIU have endorsed de Blasio, while the Retail, Wholesale and Department Store Union has backed Quinn.