Suspended senator Mike Duffy has started his defence against 31 charges of fraud, bribery and breach of trust in a packed Ottawa courtroom on Tuesday.

"I am not guilty, Your Honour," Mr. Duffy told Judge Charles Vaillancourt.

The Crown and the defence will make their opening arguments on the first day of trial, before witnesses are called in coming days.

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There are 41 days of court time set aside to decide the guilt or innocence of Mr. Duffy, but a parallel political trial will be judged outside the courtroom. The crucial evidence in that case will be about Mr. Duffy's valued work as a partisan Conservative operative, and how that affected his treatment.

The two trials are very different. The Crown prosecutors seeking to convict Mr. Duffy on 31 charges of fraud and breach of trust are not trying to show that Prime Minister Stephen Harper knew about the alleged misdeeds. Much of their work will be to lay out the rules of Senate budgets, and then to try to prove Mr. Duffy defrauded them.

The political fallout, meanwhile, depends on by-products of the criminal trial: perhaps occasions when expense claims are linked to partisan work for the Conservative Party of Canada, or peeks into the workings of Mr. Harper's PMO at times when ethical concerns took a back seat to party interests.

For the Conservatives, the danger is in evidence that suggests the government was trying to protect a political asset. Mr. Duffy was a valuable celebrity fundraiser who raked in cash for the party through recorded messages and events, and played host to the Prime Minister for fake interviews at what the Tories like to call "message events."

And when Mr. Duffy's expense scandal first blew up, when allegations emerged that he claimed a housing allowance for his home in Ottawa although he had lived in the city for decades, the Conservatives' first instinct was to protect him.

Already, opposition politicians are stoking the idea that the Conservative leadership looked the other way because Mr. Duffy was a useful partisan operative.

Liberal Leader Justin Trudeau argued at a press conference last week that Mr. Harper's team rushed to keep Mr. Duffy's expense scandal quiet to "protect someone who was very useful as a source of revenue."

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That kind of attack is always the most dangerous for any government, if it sticks, because it undermines trust in its motivations: it suggests the government put its own interests before the public interest.

In the opposition's wildest dreams, this trial would produce a smoking-gun moment, at least in political terms. Nigel Wright, who was Mr. Harper's chief-of-staff at the time, wrote a $90,000 cheque to help Mr. Duffy pay his expenses, and to cover up the scandal.

Mr. Harper has insisted he did not know of the payment, but NDP Leader Thomas Mulcair has expressed skepticism, citing e-mails written by Mr. Wright that suggested plans for a negotiated settlement of expense issues with Mr. Duffy were "good to go from the PM." But while the opposition dreams of testimony that shows the Prime Minister knew more, after repeated denials, that seems hard to imagine.

What would be just as damaging would be a regular drip of revelations that link Mr. Duffy's expenses to partisan Conservative work. The former TV journalist travelled widely to speak at fundraisers and party events.

Mr. Duffy has in the past said his partisan work is relevant: he asked the Conservative Party to pay his questioned Senate expenses, saying he incurred them doing party business. The PMO responded by trying to get Mr. Duffy out of trouble.

It is unknown how much more of that politically damaging narrative will come out in a fraud trial, which typically includes swaths of dry, complicated evidence. Prosecutors are likely to ignore most political machinations as they focus on financial ones; it is Mr. Duffy's defence that might highlight what the PMO knew to bolster the argument that the expense claims were approved or accepted.

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Perhaps there will not be new revelations, and the Conservatives will distract from the trial with budget announcements. But the political outcome is likely to depend on what we hear about Mr. Duffy's value to the Conservative Party, and how much that drove the desire to protect him, because that may colour how voters judge the government's motives.