He's facing a backlash from labour leaders after Liberals complained earlier this week about parliamentary office space provided by the NDP to their staffers' union local.
Three union leaders have sent angry letters to Trudeau, accusing the Liberals of unfairly dragging the labour movement through the mud in order to score partisan points against the NDP.
And they're questioning the sincerity of Trudeau's commitment to labour rights.
To frame the existence of the union local office on Parliament Hill "as somehow illegitimate is not only partisan and unjust," wrote Paul Meinema, president of the United Food and Commercial Workers Canada.
"It also sends a message from the Liberal Party of Canada that unions and the labour movement can be abused ... as cannon fodder in a partisan political battle."
While labour expects union-bashing from the governing Conservatives, Meinema added it's "certainly not expected from a Liberal opposition party that, in advance of the next election, has pronounced its commitment to labour rights, the union movement and fairness for working people."
Trudeau has been making a concerted effort of late to woo support from the labour movement, which has traditionally been more aligned with the NDP.
In a speech to firefighters on Monday, he promised a Liberal government would repeal "anti-union" legislation introduced by Prime Minister Stephen Harper's Conservatives and offered effusive praise for unions, which he described as "one of the few remaining forces that fight effectively for the fair wages that Canada's middle class needs."
But only a few days later, union leaders were infuriated by comments from Liberal House leader Dominic LeBlanc, a member of the secretive board of internal economy which polices the use of parliamentary resources by MPs.
LeBlanc called it "very bizarre" that the NDP are "using taxpayers' money to fund their union operation out of their leader's office." The arrangement "completely blurred the lines" about the proper use of parliamentary resources, he said.
A Conservative MP later lodged an official complaint against the arrangement with the board of internal economy.
The NDP is the only federal political party whose staff is unionized. As part of its collective agreement with staffers, the party has provided office space on Parliament Hill since 1997 for the union local.
In separate letters to Trudeau, the leaders of the UFCW, United Steelworkers and Canadian Union of Public Employees argue that it's routine for union locals to be provided office space in the workplaces of those they represent.
CUPE national president Paul Moist said the arrangement is aimed at fostering timely and productive labour relations and is a common feature of collective agreements in the public sector, as well as the private sector.
"We have encountered no partisan critiques of this labour relations practice such as a member of your caucus has launched," Moist added.
The House of Commons provides office space in the parliamentary precinct for other union locals, including the Public Service Alliance of Canada.
Steelworkers national director Ken Neumann said LeBlanc's comments betray "a total lack of understanding of the most basic principles of trade union representation."
"We are very concerned that he would choose to attack a basic trade union right that has been freely negotiated and, in so doing, attack the legitimacy of the trade union movement."
Neither LeBlanc nor Trudeau's office responded Friday to a request for comment on the union leaders' letters.
In Regina, NDP Leader Tom Mulcair made no apologies for providing office space.
"Like all unionized workplaces, the employer does provide space for the union. That's been provided for years and years under our collective agreement. There's never been any secret about that," Mulcair said.
He professed to be "terribly surprised" that the Liberals would question the practice, characterizing it as an "attack labour's right to organize and represent workers."
Since last year, the NDP's more than 600 staffers have been represented by UFCW local 232. They were previously represented by the Communications, Energy and Paperworkers union, which merged with the Canadian Auto Workers in 2013 to form Unifor.
Follow @jmbryden on Twitter
Also on HuffPost