Liberal Emmanuel Dubourg, a former member of the Quebec legislature, defeated NDP candidate Stephane Moraille, a lawyer and former singer with the band Bran Van 3000, in the Liberal stronghold.
The NDP denies any wrongdoing in the riding formerly represented by current Montreal Mayor Denis Coderre.
Liberal MP Marc Garneau wrote to the commissioner of Canada elections, citing media reports of MPs from the NDP caucus using their mail privileges to send flyers promoting the NDP to households in the riding.
He argued that if the flyers were delivered after the byelection was called, the value of the mailings would constitute illegal donations to the NDP candidate.
Garneau calculated the market value of such a first-class mailing to 400,000 households at about $20,000, or 20 per cent above the allowable spending limit for a candidate in the byelection.
He said if Elections Canada finds the flyers were sent after the writs were dropped, the value of the mailing should be added to Moraille's declaration of campaign expenses.
"As new byelections will have to be called soon in currently vacant federal seats, I feel it is essential for Elections Canada to fully investigate this matter to ensure that there is no question of taxpayer-funded resources being used to support candidates in any future elections," Garneau wrote.
Greta Levy, an NDP spokeswoman, says the party did nothing wrong.
"The rules were respected," she said.
Levy said some MPs sent messages to the riding comparing the style of leadership by NDP Leader Tom Mulcair to that of Prime Minister Stephen Harper but no flyers compared Moraille to Dubourg.
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