Mulcair told union activists gathered in Toronto that New Democrats are the only ones who can be trusted with protecting the rights of working Canadians.
The union members were attending a weekend-long "political action conference" _ it was organized by the Canadian Labour Congress as a way of bringing more attention to labour issues.
He says the labour movement shouldn't fear his wait and see stance on a Canada-European Union free trade deal, promising he won't give it his blessing if it costs Canadian jobs.
Mulcair parted company with some of the NDP's traditional allies this month when he urged them to wait to see the fine print of the deal being negotiated before calling it unacceptable.
During his Saturday speech Mulcair rattled off a list of changes the Conservative government is making that he says undermine unions and workers, adding he's counting on labour's support in the next federal election.
"We will fight. We will work with you," Mulcair said.
"The NDP can be trusted, we can be counted on and we can do this together in 2015."
Mulcair attacked a Tory private members bill that would force unions to public disclose their finances, earning a roar of support from the crowd.
He mentioned his visits to several labour flashpoints — including a locomotive plant closure in London, Ont., and a dispute over temporary foreign workers in Prince George, B.C. — as proof of his closeness to union concerns.
"I was on the picket line," he said.
He also took a dig at the Liberals, saying they've offer discount fares on Toronto-based Porter Airlines for party members travelling to next month's leadership gathering in Ottawa next month despite the fact some of airline's ground staff are involved in a strike.
Mulcair did not focus just on organized labour, touching on the raising of Old Age Security and what he called treatment of Employment Insurance recipients "like criminals" under a federal crackdown.
Several Toronto-area NDP MPs were also in attendance, and like Mulcair received a welcome reception from union members.
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