The long-anticipated third instalment of the Mike Duffy trial will begin this week with some key testimony from Nigel Wright, Stephen Harper's former chief of staff.

Summoned to testify on the question of whether the suspended Conservative senator received a bribe (which the defence denies), Wright is expected to be a crucial witness for the Crown.

Duffy has pleaded not guilty to 31 charges of fraud, breach of trust and bribery related to expenses he claimed as a senator and later repaid with money from Wright.

Wright's high-profile appearance comes in the second week of a lengthy campaign ahead of the Oct. 19 election.

During a campaign stop Sunday, Harper was asked about the trial, which resumes Wednesday — and in particular about what Wright meant when he wrote in a February 2013 email that he'd been given a "good to go" from the prime minister.

Stephen Harper

Conservative Leader Stephen Harper, asked about Wright's expected appearance during a campaign stop in Ottawa Sunday, said he'll 'let the court do its work.' (Sean Kilpatrick/Canadian Press)

"I did not know that Mr. Wright had made a payment to Mr. Duffy," the Conservative leader replied. "As soon as I learned that, I made that public. And Mr. Wright has been clear about that. This is the purpose of the process and those who are responsible and I'll let the court do its work."

And what about "good to go"?

"The words you're quoting are not my words. They're somebody else's," Harper said.

Wright has already publicly acknowledged he gave Duffy a cheque for $90,000 in order to cover his controversial spending habits while sitting in the upper chamber.

Wright claimed his only motive was to ensure taxpayers were not on the hook for the money.

Bribery charge for Duffy, but not Wright

Some observers are questioning the Crown's approach even before Wright testifies, wondering first and foremost, how Duffy could be charged with bribery but not Wright.

Joseph Neuberger is a criminal defence lawyer and senior partner at Neuberger and Partners LLP in Toronto.

He thinks the bribery charge is "one of the weaker allegations of the Crown's case," he said in an interview. 

It implies there was some kind of quid pro quo that may be hard to prove, he said.

Duffy Trial Charles Vaillancourt

Justice Charles Vaillancourt, above, has a lot more testimony to hear before deciding whether Duffy is guilty of fraud, breach of trust and bribery. (Sean Kilpatrick/Canadian Press)

"[The Crown] has to establish there was some benefit for Duffy or for the government," he said. "And if the idea is there was supposed to be an explicit understanding that Duffy was going to be quiet about this and that this is not going to go public, then I'm assuming Nigel Wright is engaging in some conduct that would draw culpability."

That doesn't square with the absence of charges for Wright, he says. Neuberger said he doesn't expect the bribery charge to stick, although he stressed that he doesn't know what evidence the Crown might have at its disposal.

"I'm not seeing it," he said. "I'm looking at it very hard, and I'm not seeing it."

Duffy's lawyer, Donald Bayne, would be best served by taking a "surgical" approach in the cross examination of Wright, Neuberger suggests.

"[Bayne should] establish there was no benefit to Mr. Duffy engaging in this transaction and to try and leave it at that," he said. "In other words, just to establish the elements, without really a broad attack on Mr. Wright."

Blaming Wright, PMO risky

Still, there could be dangers ahead for the defence in this cross-examination, according to Michael Spratt, a criminal lawyer and partner at Abergel Goldstein and Partners LLP in Ottawa.

He warns Bayne should not approach Wright too forcefully on the issue of Duffy being the victim of a scheme cooked up in the Prime Minister's Office.

"I think that that's a very risky strategy," he said. "When you start throwing mud at the [PMO] and at Wright, saying this is a giant conspiracy and there's nefarious things going on behind the scenes, that, to some extent, links Duffy to this conspiracy and these nefarious things, and some of that mud might splash back on him."

The safer route for the defence, Spratt argues, is to play on the fact that Wright was never charged and take him at his word.

"[Bayne should say] 'You're right. Nigel should never have been charged ... Nigel didn't give a bribe, and Duffy didn't accept a bribe. This was all a favour to make the taxpayer whole,'" Spratt said.

The problem with this strategy, however, is that Duffy has already gone down the road of blaming the PMO, Spratt said.

"Duffy's statements that he made at various times, including his now famous or infamous speech that he made in the chamber of the Senate, is material that Duffy can be cross-examined on," he said.

In that speech to the Senate, in October 2013, Duffy blamed the prime minister's staff for orchestrating what he called a "monstrous political scheme."

Reasonable doubt

Some observers have argued that the first leg of the trial appears to have favoured the defence.

Spratt says that's because while the Crown presented a strong "common sense" argument, Bayne nevertheless managed to sow reasonable doubt about the charges of fraud.

Spratt points to Bayne's skillful handling of the residency issue as a good example of that.

'The defence has been able to establish that there was some ambiguity with respect to the rules everyone else was operating under.' - Michael Spratt, criminal lawyer

"Bayne has drawn on other sources, the Income Tax Act, some other rules, the forms that Duffy submitted, to sow some doubt into what the rules actually were at the time," he said.

"And when you look through the Crown's witnesses, the defence has been able to establish that there was some ambiguity with respect to the rules everyone else was operating under."

Neuberger agrees reasonable doubt could get Duffy off the hook on some or all of the charges, even if the judge, Justice Charles Vaillancourt, has sympathy for the Crown's argument that it's simply common sense that some of Duffy's expense claims were inappropriate.

Senate-duffy trial

Duffy's defence team has successfully established reasonable doubt that there was clarity about spending rules in the Red Chamber, above, says one Ottawa lawyer. (Adrian Wyld/Canadian Press)

Still, Neuberger adds, many shoes have to drop before Vaillancourt can make a final determination. 

"So, we're not anywhere near a judge being able to make a conclusion, but based upon a common sense analysis, from some of these expenditures, they do raise your eyebrows," he said.

"Whether it can reach the threshold of proof beyond a reasonable doubt, given the state of what the rules were for expenditures in the Senate, I think that's a very strong argument for the defence."

The trial still has to hear from not only Wright but possibly from other members of the Prime Minister's Office and, of course, from Duffy himself, who has signaled he will take the stand. His testimony will also be significant in that it will shed light on his intentions, which, Neuberger says, are a key factor in proving fraud.

Duffy's testimony likely won't happen before the trial's last leg, which is now scheduled for November, after the election.

With files from The Canadian Press