OTTAWA -- Call it the Rathgeber clause: Liberals in the Senate say a Conservative bill that would force unions to disclose their expenses should match another Tory bill on disclosure of public sector salaries.

Conservative backbencher Brent Rathgeber quit the party caucus earlier this month after his private members' bill on public sector salary disclosure was effectively gutted by the government.

Rathgeber wanted the publication of all federal salaries of more than $188,000 a year, including employees at Crown corporations such as CBC.

But the Prime Minister's Office stepped in and raised the legislation's salary disclosure bar to over $444,000 -- prompting Rathgeber to quit the Conservative caucus.

Now, Liberal senators say if that threshold is what the government wants for public servants, why not for unions as well?

Liberal Sen. Pierrette Ringuette has proposed an amendment to Bill C-377 that would see union salary disclosure requirements raised to $444,000 from the current $100,000 in Conservative Russ Hiebert's private members bill.

"I signed these amendments because I know the government will approve of these amendments," she told the Senate late Monday.

Hiebert introduced his union bill in 2011, arguing that public disclosure of union expenses and salaries ought to be mandatory because union fees are tax-deductible.

The bill seeks to amend the Income Tax Act to require unions to give detailed financial filings to the Canada Revenue Agency, which would then make the information public in the same way it does for charitable organizations.

But there has been surprisingly wide opposition to the bill from all corners. Though it passed the House of Commons in December, several Tories voted against it, including Rathgeber.

Rathgeber argued that while union members have a right to know how their dues are being spent, he wasn't convinced that information needed to be public.

Unions say members can already access the information and the new legislation would cost millions to implement; the Canadian Labour Congress set a price tag of between $32 million and $45 million a year just to operate.

Ringuette outlined all the criticism the Senate has heard, from the privacy commissioner who warned against the bill's privacy violations to provincial government officials who said it intrudes on their jurisdiction.

"The truth is that the courts will overturn this bill, so why continue pushing it? The courts will overrule it; they will not accept this bill; so why should the Senate of Canada, the chamber of sober second thought -- or are we? -- in view of all the evidence that we have had before us?" she said.

"It is maybe for the sake of petty politics, which are beyond this institution and should always be."

Conservatives in the Senate have raised questions as well.

The Senate committee charged with studying the bill -- a committee chaired by Irving Gerstein, who also chairs the Conservative Fund Canada -- formally questioned its constitutionality, said it is concerned about implementation costs, and said the legislation is so vague that it's not clear to whom it applies.

And Tory Sen. Hugh Segal has spoken out, calling the bill bad public policy and questioning the wisdom of dispatching revenue agents to monitor its implementation.

Segal introduced an amendment of his own Monday, seeking to further narrow the bill by, among other things, excluding from it labour groups with fewer than 50,000 members.

In introducing his proposed change, Segal suggested the bill went against Canadian conservatism.

"Conservatism in the Canadian Tory context is not about the protection of class or the oppression of labour by capital or capital by labour; it is about a freedom tied to mutual respect, whatever legitimate disagreements, between all the participants in the mixed free-market system," Segal said in the Senate on Monday night.

"This bill before us, whatever may have been its laudable transparency goals, is really -- through drafting sins of omission and commission -- an expression of statutory contempt for the working men and women in our trade unions and for the trade unions themselves and their right under federal and provincial law to organize."