Conservative Party Leader Stephen Harper greets supporters at a campaign event in London, Ont., on Saturday. ((Tom Hanson/CP))

Conservative Leader Stephen Harper needs to explain his comments on a tape that is at the centre of a defamation suit launched by Harper against the Liberals, party leader Stéphane Dion said Saturday. 

Harper has said the tape, a short interview he did with B.C. author Tom Zytaruk in 2005 about Independent MP Chuck Cadman, was doctored. Based on the book Zytaruk wrote, the Liberals claimed Harper knew that two senior Conservatives attempted to bribe Cadman to vote against the Liberals in 2005.

Harper sued the Liberals for defamation in 2008 based on comments on their website about the situation with Cadman.

Harper said the tape had been edited, but a court-appointed expert in the defamation case reported Friday that the key portions that the prime minister had contested contain "neither physical nor electronic splices, edits or alterations."

Harper needs to explain "what he meant when he said on the tape that Conservative officials offered Mr. Cadman financial considerations for his vote," Dion said.

That tape includes Zytaruk's question about whether Harper knew about a $1-million insurance policy that unidentified Conservatives had allegedly offered to Cadman in return for voting against the Liberals.

"I don't know the details, I know that, um, there were discussions," Harper said. The Conservatives who approached Cadman believed there were financial issues, he said, and "Of the, uh, uh, the offer to Chuck was that, it was only to replace financial considerations he might lose due to an election."

Harper has denied saying he was unaware of the "details," and has said he confirmed only that the Conservatives offered Cadman "financial considerations" to rejoin the Tories and vote against the Liberals.

On Saturday, Harper told the CBC's Paul Hunter, "this tape has been edited and the conversation is incomplete, and you know … the Liberal party's got a big problem in court."

NDP Leader Jack Layton said the expert's finding could erode confidence in Harper.

Cadman died of cancer shortly after the 2005 vote.