The former cabinet minister raised the issue Tuesday, joining the gang-up on Trudeau by some of his competitors as the race nears its final month.
Montreal MP Marc Garneau has taken repeated direct shots at Trudeau over the past two weeks, accusing him of being an untested, inexperienced rookie who is offering little more than empty platitudes. Garneau has also challenged Trudeau to a one-on-one debate.
Former MP Martha Hall Findlay has also gone after the front-runner, questioning how he can pretend to understand the needs of middle-class Canadians given his privileged upbringing as the eldest son of former prime minister Pierre Trudeau.
She was booed for that jab at the last all-candidates debate and quickly apologized.
Cauchon joined the fray Tuesday, arguing that speaking to charities and other non-profit entities like school boards should be part of an MP's regular duties.
Hence, he said it was "clearly a mistake" for Trudeau to accept speaking fees from not-for-profit groups, an error in judgment he attributed to the front-runner's "lack of experience." And he called on Trudeau to reimburse the money.
"It's not illegal but, for non-profit organizations, I find it hard to believe he charged (fees)," Cauchon said in an interview.
He noted that Trudeau missed some votes in the House of Commons while he was delivering paid speeches, "so there's a question of double dipping there" as well.
Cauchon also seized on remarks Trudeau has made regarding Quebec secession and Quebec's refusal to sign the Constitution — since clarified — to bolster his case that Trudeau is inexperienced.
"Every time he says something, he's opening the door (to criticism). It's so easy," Cauchon said.
Trudeau has disclosed that he's earned $277,000 in public speaking fees since becoming an MP in 2008.
Trudeau ally Dominic LeBlanc pointed out that all the front-runner's speaking engagements were cleared by the independent parliamentary ethics watchdog, Mary Dawson. He argued that her judgment should be taken more seriously than "cheap shots" by leadership rivals.
Cauchon has evidently decided the only way to get attention for his low-profile leadership campaign is to join "the band of negative warriors" attacking the front-runner, added LeBlanc, a New Brunswick MP.
"Martin has probably concluded that the only air time that some candidates get is when they decide to go negative on the guy most likely to win," he said, accusing Cauchon of "artificially inflaming and distorting" Trudeau's words.
"I'm disappointed. I've known Martin for a long time. He's a friend of mine. He spoke at my wedding."
The recent barrage of attacks from rivals hasn't visibly slowed down what appears to be a Trudeau juggernaut. He won an endorsement Tuesday from Scott Brison — the 21st of 35 Liberal MPs to jump on the front-runner's bandwagon.
Garneau has been endorsed by three MPs. None of the other six contenders has snagged any endorsements from MPs, although Joyce Murray picked up support Tuesday from a veteran Liberal senator, Celine Hervieux-Payette.
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