Suspended Senator Mike Duffy will go to trial on fraud and breach of trust charges next spring – setting the stage for an explosive case months before the next scheduled federal election.

Prime Minister Stephen Harper could be among the witnesses called as Mr. Duffy fights 31 charges related to his Senate expenses and spending, including a deal in which Mr. Harper's former chief of staff, Nigel Wright, repaid $90,000 in claims on Mr. Duffy's behalf.

Mr. Duffy's lawyer, Donald Bayne, made a court appearance on Tuesday in Ottawa after agreeing to a trial date with the Crown. The judge accepted the dates and set a 41-day trial before a judge for April 7 to May 12, and June 1 to June 19 in the Ontario Court of Justice.

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Mr. Bayne also sought to enter a plea of not guilty, but the judge said the appearance was not the time to do so.

"As we've said from the start, we trust that the evidence will show that Senator Duffy is innocent of these criminal charges," Mr. Bayne told reporters outside court. He declined to answer further questions, and Mr. Duffy did not attend the hearing.

RCMP laid 31 charges against Mr. Duffy in July after a lengthy investigation stemming from the payment by Mr. Wright. Of the 31 charges, two relate to his residence, 18 to his expense claims, eight to contracts he awarded from his Senate budget and three to the deal with Mr. Wright.

Mr. Duffy has pressed to go to trial as quickly as possible, with his lawyer saying the suspended senator's health is a factor in the push for a quick resolution.

In a court appearance last week, Mr. Bayne declined to rule out calling Mr. Harper as a witness, saying all options were being considered.

Mr. Harper, who has in the past claimed parliamentary privilege to avoid testifying in a case, has said he sees no reason he would be called to testify this time. He has repeatedly disavowed knowledge of the plan for Mr. Wright to repay Mr. Duffy's expenses, although RCMP documents filed in court during the investigation allege several members of his inner circle were involved.

"I think it's of great interest to all Canadians that the trial will be held this spring," NDP Leader Thomas Mulcair, who has grilled Mr. Harper in Question Period over his role in the scandal, said on Tuesday. "I think that we're finally going to see whether Mr. Harper's going to hide behind parliamentary privilege or he's going to come and testify because there are certainly going to be questions for him in that affair."

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Liberal Leader Justin Trudeau hopes Mr. Harper testifies "so we can actually get to the bottom of what happened in this sordid affair," but added: "My expectations of that happening are fairly low."

MPs can claim parliamentary privilege, although House of Commons rules say the power is "not intended to be used to impede the course of justice and, therefore, is regularly waived, particularly for criminal cases." MPs are also typically exempt from attending court when the House of Commons is in session.

Mr. Harper has claimed privilege once before since becoming Prime Minister, when sued by former Conservative candidate Alan Riddell. As Prime Minister, Mr. Harper has testified once, in a case he filed against the Liberal Party. Claiming privilege could lead to separate legal wrangling that would likely delay the trial.

A spokesman for Mr. Harper declined to comment on the trial date, but said the government remains committed to the legislated election date of Oct. 19, 2015. "There is no plan to change the timing of the election scheduled for fall 2015," spokesman Jason MacDonald said. Mr. Duffy was suspended from the Senate last year, and is eligible to reclaim his seat and salary after the election.