That's one of the questions crystallizing now that Duffy's trial has begun to hear from his former Conservative caucus mates.
He's pleaded not guilty to 31 counts of fraud, breach of trust and bribery, with more than half of those charges related to trips he billed to the Senate.
The Crown says Duffy was attending political events, and not dealing with Senate business.
Duffy appears to have a two-track defence. He argues the Senate never explicitly prohibited travel to political events in its rules and that he was, in fact, doing official business while on these trips.
The latter argument is key, because it matches what other ministers, MPs, senators and the prime minister himself do on a regular basis — they fly to an official event and then tack on political stuff.
British Columbia Conservative MP Ron Cannan, who testified Thursday at the trial, explained the scenario when talking to reporters outside the courtroom.
"I was in city council for nine years, and we had a really good senator, Liberal senator Ross Fitzpatrick, and he used to bring senators in, they might meet with mayor and council for an hour and whatever they did afterwards, if they went to his winery and toured and played golf, they could incorporate that," Cannan said.
In Cannan's case, he heard that Duffy was going to speak in the neighbouring B.C. riding of MP Cathy McLeod, and asked if the senator could add another Conservative mingle in June 2009.
Duffy was on a swing through the province and the Yukon, attending Conservative events with other MPs and senators.
Duffy's daily calendars, which are now court exhibits, show a sprinkling of non-partisan events around that time, including a speech to the Fraser Institute entitled "Are the Conservatives being Conservative?" a meeting with local mayors in Duncan's riding and a meeting with the Chamber of Commerce in Whitehorse.
The court heard earlier in the week that MP John Duncan had specifically invited Duffy to speak in Courtenay, B.C. At both his and Cannan's fundraisers, the local Conservative riding associations picked up hotel bills and some meals. Neither MP knew who paid for Duffy's air travel.
A former Conservative colleague of Duffy's, who asked to remain anonymous, said it's common practice for parliamentarians to organize an official meeting with someone in order to help pay for a trip of personal or party interest.
"The age-old excuse is you go in for an announcement for something and then you stay for the fundraiser," said the Conservative.
What is abundantly clear is that Duffy was a big draw among Conservatives who wanted to raise money or just interest among local supporters. An analysis of his daily calendars shows he helped out at least 74 former and current MPs with political activities over four years.
"He was high profile ... probably one of the highest-profile members of the Senate, because of his years in the media, and people would have an opportunity to come and listen to him," Cannan said under questioning by Crown prosecutor Jason Neubauer.
"He's an entertaining speaker with lots of stories, and it created an opportunity for dialogue with our members."
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