OTTAWA - Stephen Harper is escalating his lawsuit against federal Liberals in a bid to muzzle legitimate criticism about his role in the Cadman affair, the Liberal party says.

The charge is part of the Liberals' legal counter-punch, filed Friday in response to the prime minister's recent decision to add another $1 million claim - for "misappropriation of personality" - to his $2.5 million defamation suit against the party.

"This claim by Prime Minister Harper is an attempt by the governing party to silence the Official Loyal Opposition on a matter of significant public interest pertaining to the government of Canada," the Liberal party says in the statement of defence filed with the Ontario Superior Court of Justice.

The party argues that Harper's claim "is fundamentally incompatible with the right to freedom of political expression" enshrined in the Charter of Rights and Freedoms.

The lawsuit stems from a fundraising email and statements posted on the Liberal website last February which asserted that Harper knew of a Conservative attempt to bribe Chuck Cadman, the late Independent MP, in return for his support during a crucial 2005 confidence vote.

Among other things, the Liberal missives accused Harper of condoning "immoral," "unethical" and "illegal" behaviour.

The Liberal accusations were based on a tape recording of an interview with Harper by Cadman biographer Tom Zytaruk. Harper is heard saying he was aware of an offer made to the MP "to replace financial considerations that he might lose due to an election."

Harper launched a $2.5 million defamation suit in March. After mandatory mediation failed to produce an out-of-court settlement, he amended the suit early last month to add a $1 million claim for misappropriation of personality.

At the same time, Harper sought an injunction against the Liberal party's use of the Zytaruk tape. He contends the tape was doctored to leave a false impression he knew Conservative officials had offered Cadman a $1 million life insurance policy - as Cadman's widow, Tory candidate Dona, alleges in Zytaruk's book.

In a television interview Thursday, Harper defended his decision to up the ante.

"I don't think you accuse people of being criminals when you don't have proof. That's a serious violation of the law," he said.

In his amended statement of claim, Harper's lawyer argues that the Liberals "have used the edited and doctored audio tape to wrongly usurp the plaintiff (Harper) of his right to control his own image and to portray the plaintiff in a false light."

Furthermore, the statement asserts that the Liberals have exploited the doctored tape "by embarking on a campaign of vilification" of Harper.

Harper's amended claim does not directly accuse the Liberal party of tampering with the tape. But it does note the party has disseminated the tape through links on the party website, press conferences and a recent stunt by Young Liberals, who broadcast Harper's words from a van driven through downtown Ottawa.

"The defendants were informed that the edited and doctored audio tape was incomplete, edited and not credible but consistently ignore this fact," Harper's lawyer, Richard Dearden, says in the amended statement of claim.

He says the Liberal party has continued to make "defamatory statements" about Harper with "reckless disregard for the truth."

In its counter-statement of defence, the Liberal party questions why Harper's lawsuit targets only the official Opposition, even though the Zytaruk tape is readily available on YouTube and various news media websites.

The party also refuses to accept that the tape was doctored and says suggestions that Liberals were in any way involved in tampering with it are "outrageous, scandalous, politically motivated, completely without foundation and expressly denied."

If Harper's reputation has been damaged, the party contends "it is as a direct result of Prime Minister Harper's own conduct." It argues that Harper has repeatedly failed to explain the context in which his taped remarks were made, what part of his remarks were edited out and what he meant by financial considerations offered to Cadman.

The party contends that Harper's claim is a SLAPP suit, "strategic lawsuit against public participants," launched to prevent the Liberals from drawing voters' attention to "alleged improprieties within the current government."