Prime Minister Stephen Harper's legal tactics are likened to those of an authoritarian state in court documents filed recently by the Liberal party.

The comparison is made in an affidavit the Liberals filed in defence against a $3.5-million defamation suit Harper launched earlier this year.

The Liberals have also served notice to the Ontario and federal governments they intend to question the constitutionality of Harper's lawsuit on grounds it is an attempt to muzzle the opposition.

The latest Liberal court filings include an affidavit from University of Toronto professor Peter Russell that argues no Canadian prime minister has sued political opponents for libel despite a history of dramatic accusations during stormy debates in Parliament.

"This use of legal action to silence the opposition is characteristic of authoritarian governments," Russell says in an expert opinion obtained by Toronto lawyer Chris Paliare, who is representing the Liberals.

"It is incompatible with democratic government," argues Russell, a political scientist and professor emeritus at the university.

The reference to authoritarian rule and the rare constitutional challenge of Harper's libel action revolve around his request for a court order to prevent the Liberal party from using or distributing copies of an audio tape at the centre of the libel suit.

The tape contains an interview a British Columbia author conducted with Harper in 2005 about attempts Conservative party operatives made earlier that year to persuade late independent MP Chuck Cadman to help defeat the minority Liberal government in the Commons.

'Financial considerations'

Harper is heard telling author Tom Zytaruk he was aware Conservatives had discussed "financial considerations" with Cadman as they tried to coax him into opposing the Liberals during a confidence vote.

In a subsequent biography, Zytaruk quoted Cadman's widow as saying the terminally ill Cadman told her the Tories had offered him a $1-million life insurance policy to vote against the Liberals.

Harper sued the Liberal party last March after it posted headlines on its website suggesting he was aware of attempted bribery in the affair. Harper maintains he was referring to potential election campaign expenses when he spoke to Zytaruk.

Russell's affidavit was partly a response to an amendment Harper made in June to his claim against the Liberals, adding $1 million to the original $2.5 million in damages because the Liberal party had engaged in "misappropriation of personality."

That claim was based on two audio experts Harper's lawyer retained, who analyzed the Zytaruk tape and concluded it had been doctored. A third said laboratory examination was required to conclusively verify the tape's integrity, and an Ontario Superior Court judge has ordered Zytaruk to surrender the tape for more tests.

Russell, supported with evidence filed in a separate affidavit from a Liberal party computer expert, said the tape is accessible on several news media websites. He also referred to the "virulent and sustained" media attacks the Conservatives have mounted against Liberal Leader Stéphane Dion over the past year.

"It would be a gross interference with freedom of political debate in Canada and grossly unfair if the courts were to interfere and by way of injunction prevent the official Opposition party from using material already fully available in the public realm," Russell said.

Obtained copies on the web

The party also argues in court submissions that it obtained copies of the audio tape from media web sites.

A spokesman for the Conservative party dismissed Russell's affidavit, saying the professor is a "close friend and fishing buddy" of Dion's.

Spokesman Ryan Sparrow said he was basing the description on published statements in news commentary earlier this year.

"Peter Russell is not an audio expert and his opinion is irrelevant," Sparrow said.

Paliare acknowledged Russell has gone fishing with Dion and "considers him a friend" but added Russell also goes skiing with University of Calgary political scientist Tom Flanagan, a friend and mentor of Harper's, and is also a friend to Health Minister Tony Clement, who was once a student of Russell's.

The Liberal plan to challenge the constitutionality of the libel suit is based on the argument it is a legal action by Harper in his political role as prime minister rather than as a private citizen, Paliare said.

He cited the government's decision to assign MP James Moore, a parliamentary secretary, to hold a news conference denouncing the Zytaruk tape as evidence.

The first hearing in the case is scheduled to begin Sept. 22 in Ottawa.