The International Longshore and Warehouse Union Local 142 Thursday announced its endorsement of Kirk Caldwell in the race for Honolulu mayor, giving the former city managing director the lion’s share of backing from organized labor.

Caldwell already has won the backing of the Hawaii Government Employees Association, the United Public Workers, the State of Hawaii Organization of Police Officers, the Hawaii Fire Fighters Association and a handful of smaller unions.

With 18,000 members, ILWU Local 142 boasts of being the largest private-sector union in the state.

“I think an endorsement by the ILWU is extremely important because of the size of their membership,” Caldwell said at a news conference at ILWU’s union hall. “We all know that in elections unions can turn out people to sign wave, it can turn out people to mass canvas and that makes a huge difference for the grassroots, boots-on-the-ground kind of program.”

The union also interviewed incumbent Mayor Peter Carlisle and former Gov. Ben Cayetano, the other major mayoral candidates, before reaching its decision.

Officials noted all three of the candidates have been endorsed by the ILWU for various offices in the past and it interviewed all three to give members a chance to question them on current issues.

“Kirk was the most honest person as far as talking about issues other than the rail,” said Wilfred Chang, the union’s Oahu division political action coordinator. “He spoke about the rail, but there are other issues beside the rail like our sewers, our water, our failing roads, just general quality of life issues that would be city issues.”

One candidate can win outright by capturing 50 percent plus one vote in the Aug. 11 primary election. If not, the top two vote-getters face off in the Nov. 6 general election.