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Tom Mulcair and some of the NDP's traditional allies in the labour movement are at odds over his contention that appointed senators have no business blocking legislation passed by elected MPs.

The NDP leader maintains the unelected Senate has no moral legitimacy in a democracy to thwart the will of elected representatives — an argument he's ramped up in the wake of this week's scathing audit of senators' expenses.

At the same time, however, union leaders have been feverishly lobbying senators to defeat bill C-377, a private member's bill approved by the House of Commons and strongly backed by the Prime Minister's Office that would force unions to publicly disclose details of their spending.

The Senate has already rejected the bill, which Mulcair has promised to repeal should he become prime minister, once before. Back in 2013, 16 Conservative senators broke ranks to help pass amendments that effectively gutted the bill, which critics have denounced as unconstitutional, undemocratic and an invasion of privacy.

But before the Commons could consider the Senate amendments, Parliament was prorogued and, in accordance with the rules for reinstatement of bills following prorogation, it wound up back before the Senate in its original form.

It is currently stalled in the midst of third reading debate, awaiting a ruling from the Speaker on a motion by Conservative Sen. Diane Bellemare that the bill is out of order.

With the fate of the bill to be decided in the next couple of weeks, union leaders are torn between their sympathy for the NDP's campaign to abolish the upper chamber and their hope that the Senate will exercise its constitutional power to obstruct, amend or defeat the legislation.

"I'm a New Democrat ... I'd be happy if we didn't have a Senate," Paul Moist, national president of the Canadian Union of Public Employees, said in an interview.

Nevertheless, Moist added: "While the Senate exists and if it's constitutionally supposed to be a place of sober second thought, we were absolutely happy with their (rejection of the bill the first time around) and we think they should chuck this one as well."

Similarly, Hassan Yussuf, president of the Canadian Labour Congress, said it's "a separate debate" whether the Senate should continue to exist.

For the time being, "It is part of the democratic structures of our country right now and, as such, you can't ignore it because it has both a role and a responsibility in a democracy."

Although Yussuf personally believes the country can probably do without the Senate, he said important issues do arise from time to time that require the chamber to use its constitutional power to countermand the wishes of the Commons. He noted, for instance, that the Senate killed the last attempt by the federal government to criminalize abortion in 1990.

"Had it not been for the Senate, abortion would still be an illegal act for women to control their bodies," Yussuf said, adding that if senators' only job "is to rubberstamp what the House has done, then we don't need the Senate."

Unifor president Jerry Dias sees C-377 as a test, perhaps the last test, of the Senate's relevance. If senators blindly follow party orders and pass the bill — in the face of widespread opposition from five provinces, the Canadian Bar Association, unions and the privacy commissioner, among others — he said they'll be putting "just another nail" in the Senate's coffin.

"They're a Senate that's in trouble," Dias said. "The only question now becomes how much more foolish do they want to look?"

The union leaders' enthusiasm for a muscular Senate overturning the will of the elected House of Commons — at least on C-377 — is in distinct contrast to Mulcair's view.

"What moral authority do they have to make laws for the rest of us? None," the NDP leader told CBC's Power and Politics on Wednesday, maintaining that senators do "nothing useful in a democracy."