Unions call it a "witch hunt," and even some government MPs don't like it, but the Conservatives are rushing a bill through the House of Commons this week that will force unions to disclose details of their spending.

In the final days before MPs break for the holidays, the government is fast-tracking Bill C-377, which will go to a final vote Wednesday evening.

Private members' bills usually move very slowly through Parliament, but the Prime Minister's Office has had the bill amended and sent to the Senate, speeding up the late stages. The government's amendments were introduced Friday and – because of Conservative procedural moves – will be voted on Wednesday, just before the final House of Commons vote on the bill.

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Bill C-377 – presented by B.C. MP Russ Hiebert – would force unions to make a long list of financial disclosures to the Canada Revenue Agency that would then be posted online. The list of proposed information that would be disclosed includes a balance sheet of assets and liabilities, a statement of income and expenditures, a detailed list of all transactions over $5,000 (raised to $100,000 under the proposed amendments) and a limited breakdown of payments to employees, including salaries, stipends, bonuses, gifts and a record of the percentage of time dedicated to political and lobbying activities.

The Tories who favour the legislation say unions should make the disclosures because they have tax-exempt status.

Some Conservative MPs are expected to vote against the bill. Edmonton-St. Albert MP Brent Rathgeber said the amendments would improve it, but he plans to oppose it because it is based on a "fallacious" premise that receiving a tax deduction is the same as getting federal tax dollars.

Mr. Rathgeber, a former labour lawyer who represented management, said he expects other Conservative MPs to join him in voting against the bill.

"As a legislator, I'm just having a difficult time determining exactly what the public interest is in this type of legislation," he said. Mr. Rathgeber said unions are essentially private clubs like law societies or industry associations that benefit from tax deductions.

"So I just cannot accept the premise that tax-deducted union dues is somehow akin to public dollars and therefore creating a public interest," he said.

The opposition Liberals have proposed amendments that would have the law apply to a broader range of member-based organizations that receive preferential tax treatment. Mr. Hiebert, the bill's sponsor, does not support the Liberal amendments.

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Interim Liberal Leader Bob Rae said Mr. Hiebert's opposition to the amendments proves his party's point.

"What that tells you is what we wanted to show people, and that is that this is not about transparency," he said. "It's about harassing trade unions."

In an interview, Mr. Hiebert insisted that is not the case. He said he and the government have listened to the critics and have amended the bill to ensure it addresses concerns about privacy and the cost of disclosure.

"It's simply about accountability and transparency.This legislation has the opportunity to increase the confidence that Canadians have that labour organizations spend their money wisely and effectively and with financial integrity," he said.

Conservative Parliamentary Secretary Pierre Poilievre used more combative language in defending the bill in response to questions from the NDP.

"Now they realize that transparency is upon them and that we are going to find out how all of this money that these unions forcefully take from workers is spent," Mr. Poilievre said.

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Chris Smillie, a spokesman for Canada's Building Trades Unions, said critics of unions will use the law to launch a "witch hunt" for embarrassing spending items.

"It has the potential to dampen the role that unions play in Canadian industry," he said. "It's a nasty one."