As the government's proposed anti-terror bill heads towards the legislative finish line, some of its more conservative-minded critics are warning Prime Minister Stephen Harper and his caucus that its passage could carry a heavy cost for his party when it hits the campaign trail later this year.

Legislation like C-51 "will inevitably lead to a split in the conservative movement," according the letter, which was published on the StopC51 website on Tuesday morning .

Libertarian-leaning voters who have backed the Conservatives in the past may choose to throw their support behind the Libertarian Party of Canada or independent, small-c conservative candidates, it says.

"Although these might not attract large vote totals, they could drain off enough votes to deny the CPC a victory and lead to the return of the Liberals, or insert the New Democrats or a coalition into government."

More than 60 signatures

The letter has been signed by more than 60 supporters — who describe themselves as from the "conservative and libertarian side of the political spectrum" — including National Firearms Association president Sheldon Clare, Free Dominion co-founders Connie and Mark Fournier, National Post comment editor Jesse Kline and Libertarian Party Leader Tim Moen.

Clare's inclusion on the list is particularly noteworthy, given his apparent reluctance to publicly criticize the bill in the past.

Earlier this spring, he was initially slated to share his concerns over the bill with the House public safety committee, but abruptly pulled out just before his scheduled appearance, sparking anger amongst some NFA members.

Now, however, he seems to have come around to the collective view espoused in today's letter: namely, that C-51 "represents everything that principled conservatives have been fighting against for the past decade."

Major concerns about C-51 outlined in the letter include:

Enhanced information-sharing powers that will allow the government to "create databases on all Canadians with no oversight," which, the letter says, will lead to "a long-gun registry on steroids."

New provisions against the promotion of terrorism that could lead to "secret trials" and the censorship of free speech — both of which go against principles that "resonate strongly with voters on the right."

Allowing CSIS agents to employ "vague, and even unconstitutional 'disruption' measures" in applying the new rules, which could lead to the targeting of Canadian website operators.

'Valid concerns' dismissed: letter

"The problems in C-51 could have been fixed, if you had chosen to carefully listen to the advice of experts who honestly see the legislation as ill-conceived, setting dangerous precedents, and opening up doors to severe violations of Charter rights and privacy expectations of citizens in a free society," the letter says.

"Instead you chose to attack those sounding the alarm as leftist ideologues and dismiss their valid concerns."

The letter ends by posing a pointed question to the recipients: "Do you really want to live in a C-51 Canada that you don't govern?"

"We thought not," it concludes.

"We don't either. Kill Bill C-51."

The letter comes as the Senate is preparing to wrap up a final round of debate on the bill, which could be passed into law later this week.

Protests against the legislation were held in several Canadian cities again last Saturday, the third time simultaneous rallies were organized by groups concerned about the bill's provisions.

In video of the Toronto rally shot by citizen journalist Derek Soberal, lawyer Rocco Galati called it a "fascist and dictatorial piece of legislation" and announced that if it passes, he'll mount a court challenge to overturn provisions he says threaten Canadians' constitutional rights to free speech and assembly.

Galati has challenged several of the Harper government's actions, most notably winning a case which found Marc Nadon ineligible to sit on the Supreme Court. He's also mounting a challenge to recent changes to the Citizenship Act.

The Canadian Bar Association is also among the critics warning legislators about the bill's provisions.