The Conservatives say one of their final legislative moves before the Christmas break is a gift in the name of transparency, but opponents see the bill as a lump of coal.
UPDATE: The bill passed the House of Commons on Wednesday night.
Bill C-377 would amend the Income Tax Act to require unions to provide detailed annual financial filings to the Canada Revenue Agency, which would in turn make the information public.
The material would cover salaries and expenses, though amendments made to the bill would see some personal information held back for privacy reasons.
The Tory MP behind the bill said he introduced it because he believes unions ought to be subject to the same public reporting requirements as charities as they enjoy the same tax-exempt status.
Like donations to charities, union dues are tax deductible.
"I believe there is a genuine public purpose served by requiring financial transparency in all institutions that receive a substantial public benefit," Russ Hiebert said during debates on the bill.
"It exists in government, Crown corporations, charities and most recently on native reserves. Now we are extending transparency to another set of institutions that enjoy public benefits, that being labour organizations."
But union leaders and opposition politicians say the bill imposes onerous, costly reporting requirements and in many provinces, unions already report this information to the people who need it, that is, their members.
"The Conservatives want their corporate friends to have access to this information so that they can undermine unions," the United Food and Commercial Workers Canada said in its campaign against the bill.
While it's unusual for a private member's bill to become law, Hiebert's appears to have the blessing of the Conservative leadership.
Labour Minister Lisa Raitt said she is in favour of the bill as it will help inform union members before casting their ballots for leadership.
"What we're focused on is making sure that Canadians have the right information they need in order to make good decisions," she said.
The Conservatives have repeatedly been accused of using their majority muscle to overpower union rights in Canada.
They've used, or threatened, back-to-work legislation in labour disputes in the airline industry and at Canada Post, arguing the economy could not afford disruption in those services.
Earlier this year, the Conservative party complained to Elections Canada about the role unions were playing in sponsoring a New Democratic Party convention.
The NDP eventually had to pay back the money as it broke political financing rules.
Unionization rates in Canada have been gradually declining since 1997, when 33.7 per cent of employees were union members or covered by a collective agreement.
Last year, the number stood at 31.2 per cent.
The bill is an attack on the rights of those Canadians, said NDP Leader Tom Mulcair.
"This is an attempt by the Conservatives to break down the system of representation and protection of workers' rights in Canada," Mulcair said.
"It will of course be defeated by the courts but in the meantime it will require a lot of bureaucracy."
In debates on the bill before a Commons committee, the Canadian Bar Association warned the bill could violate the Constitution, as it could be interpreted as an infringement on union business and therefore the right to association.
Quebec's labour minister also sent a letter to Raitt suggesting the bill sets a precedent that runs counter to the rights Quebec has to manage its own labour relations.
Raitt said she had received the letter and would speak to the minister.
Hiebert does not appear to have the support of all his caucus MPs.
At least one Tory has expressed reservations about the bill and says he intends to vote against it.
"I do not see that non-members of unions such as myself have any interest in how the unions spend those dues," said Brent Rathgeber.
Another Tory MP, James Rajotte, said that while he's supportive of the bill, he thinks there is room for a broader conversation.
He noted that provincial colleges of physicians and surgeons or law societies are also tax-exempt organizations.
"That's part of the valid debate, but I think it's beyond the scope of this particular bill but I think that should be considered," Rajotte said.
Another major beneficiary of tax-exempt status is political parties, which have to disclose donors but not what they do with the money.
Also on HuffPost