They provide candidates with access to their thousands of members and lend them considerable organizational muscle.
They also bring moral suasion to bear on party members who still cherish the NDP's traditional ties to trade unions, which were founding partners in the party 51 years ago.
Hence, endorsements from unions, or from individual labour leaders, are coveted by leadership hopefuls.
So far, those endorsements have been spread fairly evenly among four contenders: Brian Topp, Peggy Nash, Thomas Mulcair and Paul Dewar.
"I think it's pretty split. There's no one particular candidate who's got the support of labour," says Ken Neumann, president of the United Steelworkers, which has endorsed Topp.
Neumann and other labour leaders acknowledge that the role of unions in the NDP has evolved since 2003. That's the year unions were legally barred from donating money to political parties, including leadership candidates.
And that's the last time the NDP held a leadership contest in which affiliated labour unions were guaranteed 25 per cent of the vote. There is no reserved labour share of the vote in this contest, which will be decided by a strictly one member-one vote system.
What's more, the vast majority of party members are expected to vote by mail or online well before the March 24 convention, making it virtually impossible for unions — or any other group, including the leadership camps themselves — to assemble voting blocks of delegates who'll do as their told.
"You're hopeful that your membership will follow the leadership of the union ... but you have no assurances whatsoever," says Neumann.
Indeed, while the national USW has endorsed Topp, its Toronto area council has backed Nash.
Still, unions have conducted party membership drives, urging their members to join the NDP and be eligible to vote in the leadership. They've sent out email blasts promoting their preferred candidates and held events to help showcase them.
Some unions, such as the Canadian Union of Public Employees, are sitting out the leadership contest. But individual leaders within those unions are endorsing candidates.
Fred Hahn, president of CUPE Ontario, for instance, has thrown his support behind Nash.
Given the huge membership bases unions can mobilize on behalf of a candidate, Hahn maintains: "We're as influential as we've ever been" in the current contest.
Topp, already backed by the steelworkers, scooped up a second big one Wednesday — the Communications, Energy and Paperworkers union. He's also got the nod from the Quebec branch of the Public Service Alliance of Canada.
Nash has been endorsed by the Canadian Auto Workers, for whom she used to work. She's also won endorsements from the presidents of labour federations in Alberta, Ontario, Nova Scotia, Newfoundland and Labrador and the northern territories and from past Canadian Labour Congress president Bob White, among others.
Mulcair has won the backing of the United Food and Commercial Workers, along with individual endorsements from several past labour federation presidents in Ontario, British Columbia and Alberta and one current president in Prince Edward Island.
Curiously, Topp, Nash and Mulcair each lay claim to support from the country's largest private sector union, since the USW, CAW and UFCW all tout themselves as such. Each boasts more than 200,000 members.
Dewar has scored endorsements from the National Union of Public and General Employees and the International Association of Machinists and Aerospace Workers, as well as the current and three past presidents of the Manitoba Federation of Labour.
On Wednesday, Dewar picked up the support of the Winnipeg Labour Council president and a top executive with PSAC's national capital region branch.
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