Conservatives are staring down the possibility of heading into their party convention next week without a piece of legislation they'd hoped to showcase.

The Senate is mired in debate over amendments to a Conservative MP's controversial private member's bill which would force unions to publicly disclose all their financial information.

The Liberals are wielding all manner of procedural tools to stall a vote on the bill but there are Conservatives who aren't in favour of it either.

The Liberal leader in the Senate, James Cowan, said his party wants to see the bill "gutted, amended or defeated."

"And we'll stay here as long as is necessary to get that done," he said Friday as senators were preparing to leave for the day. "This is a bad bill and it should be defeated. And if, as and when it comes to a vote we will vote against it, absolutely."

Party sources tell The Canadian Press that as many as half a dozen Tory senators could flat out refuse to vote in its favour and several others are on the fence.

Conservative Hugh Segal, who has proposed amendments to the bill, said there isn't a split in the Tory caucus but rather, "there's an honest debate going on."

He said senators have a responsibility to fix a bill that he predicted is going to prompt constitutional legal challenges and problems for Canadians.

"Why would we impose upon trade unions, who are an integral part of a strong free market economy, rules that we would not be prepared to accept ourselves?" he said.

Opposition to the bill has come from all sides since it was introduced in 2011 by B.C. Tory MP Russ Hiebert.

Hiebert argued that unions ought to be subject to full disclosure because they get tax breaks and so the public has a right to know how their money is spent.

But legal experts, all manner of labour unions, provincial officials, the privacy commissioner, accountants and even the association representing police officers have raised objections to the bill.

Bill unlikely to pass before convention

The concerns saw the Conservative-led banking committee which studied the bill report back that they were worried about its cost, constitutionality and necessity, and considered those issues substantial enough to warrant further debate in the Senate.

Such debate ought to be welcome, said Liberal Sen. Paul Massicotte.

"The Senate only has merit as an unelected body if it truly acts as a chamber of sober second thought based on the merits of bills or amendments and not based on some partisan political calculation," he said in speaking against the bill on Thursday.

But the proposed law does have widespread support from grassroots members of the Conservative party, many of whom were also hoping to see the party's union policy taken further at the Calgary convention.

Resolutions on the subject up for debate include several measures governing the use of union dues and making union membership voluntary.

Meanwhile, the issue of transparency is also on party members' minds with many frustrated over a string of spending scandals that cost caucus one cabinet minister and three senators.

Having Tory senators show up at the Calgary convention having failed to pass a so-called transparency bill while the government's own transparency is under attack is likely to add another layer of tension to the event.

But at the same time, options to get a vote done by the start of the convention next Thursday are limited.

If it were a government bill, the vote could be whipped, meaning that the leader of the government in the Senate, Marjory LeBreton, could force all of the Tory senators to vote in its favour.

But private member's bill are traditionally free votes and their use also takes off the table several other tactical options which would allow the Conservatives to use their majority muscle to choke off debate and force a vote.

Amendments include threshold hike

The current debate now circles around a set of amendments being proposed to the legislation, which was passed by the Commons last year.

As written, Bill C-377 would require all unions to compile detailing financial statements which would be available for public viewing online.

The statements would have to include information expenses over $5,000 and salaries over $100,000.

The Liberals are proposing that rather than $5,000 and $100,000 thresholds the sums be raised to match those contained in another disclosure bill recently passed by the Commons.

Meanwhile, Tory Sen. Hugh Segal has proposed further amendments which would narrow the scope of the bill to see it apply only to unions of certain sizes.

Among his concerns are the way it could affect millions of mutual fund holders as well as solicitor-client privilege.

"This will actually worsen labour relations in Canada, slow economic development, and upend the balance between free collective bargaining, capital investment and return, which are vital to a strong and free mixed-market economy," Segal said in speaking to his amendments earlier this week.

"As a Conservative, I oppose the upending of this balance."

LeBreton said Friday she does not share Segal's opinion and that she has told her Conservative colleagues in the Commons that "we would do everything possible to get this bill [passed] before we adjourn."

"But of course the debate goes on and it's still before the Senate and there's lots of people who want to participate in the debate. There's some amendments we're going to have to vote on so we'll have to wait and see what happens," she said.

Adopting Segal's amendments or others however, still wouldn't get the bill signed into law this summer as it would have to be returned to the Commons.

And MPs went on their summer break earlier this week.

'Backbench spring' spreads to Senate?

The possibility of a rift in the Conservative Senate caucus over the bill follows a handful of backbench Conservative MPs rising up earlier this year demanding they be given more freedom to speak in the Commons.

It was part of a difficult spring for caucus cohesion overall.

The Tories lost cabinet minister Peter Penashue in a byelection after his resignations over issues with his election expenses.

Sen. Patrick Brazeau was kicked out of caucus after he was arrested and criminally charged in an assault matter and he was also dinged for improperly filing expense claims.

Sen. Mike Duffy was expelled over issues with his expense claims, and he was followed out the door by Sen. Pamela Wallin, whose claims are under investigation.

Backbench MP Brent Rathgeber also quit over frustrations around the tight control from the Prime Minister's Office.

with files from Canadian Press