But NDP Leader Tom Mulcair insists the party has respected all the rules and done nothing wrong.
Government whip John Duncan says the all-party board of internal economy has alerted Elections Canada to bulk flyers sent by New Democrat MPs to voters during two byelections last November, using free parliamentary mailing privileges.
Duncan says the board wants to ensure that the electoral watchdog is aware that the flyers might constitute a campaign expense in the Montreal riding of Bourassa and the Manitoba riding of Brandon-Souris.
Liberals and Conservatives, meanwhile, have asked the internal economy board to investigate the NDP's use of Parliament-funded staff to operate satellite offices in Quebec and Saskatchewan.
Taken together, the Conservatives argue the NDP has demonstrated "a pattern" of abusing taxpayer's money.
Pierre Poilievre, the minister responsible for democratic reform, used the controversy to turn the tables Monday on New Democrat MPs who accuse the government of trying to stack the next election in the Tories' favour through sweeping changes to the Canada Elections Act.
"The NDP should start coming clean about taxpayer-funded resources that it has been employing to illegally finance campaigns," Poilievre told the House of Commons.
"It should come clean about all of that illegal money. It should pay it back to taxpayers and it should admit that it represents a violation of the Canada Elections Act."
At issue are flyers sent by various New Democrat MPs to constituents in Brandon-Souris and Bourassa, calling for abolition of the Senate and slamming Prime Minister Stephen Harper's record on reforming the upper chamber. The mailings were authorized by the NDP's official agent, which suggests they were partisan material, but used free parliamentary mailing privileges.
Mulcair said the byelections had not been called when the flyers were mailed out, although he acknowledged they might have arrived in mailboxes in the ridings after the campaigns were underway.
"We checked and double checked with the Speaker (of the Commons) before going that route," he told a news conference.
Mulcair noted that both Tory and Liberal MPs have used their free mailing privileges in the past to send taxpayer-funded flyers into NDP MPs' ridings.
"So, at some point, you're only going to take it for so long and you're going to use the same rights as the others," he said.
However, the issue is not the flyers per se but whether sending them during a byelection campaign constitutes an illegal election expense.
Mulcair similarly made no apologies for using parliamentary-paid staff to run an NDP satellite office in Montreal.
"Unlike the Liberals, who think that everything takes place here in the Ottawa bubble and would prefer to keep all their staff here so that they can talk to lobbyists, the NDP does like to fan out across Canada."
The Montreal office is "doing support work for caucus, which is allowed," Mulcair added.
He did not specifically address why the NDP is setting up a similar office in Saskatchewan, where the party has no MPs.
Liberal ethics critic Scott Andrews said it's worrying that the job posting for the Saskatchewan office lists election campaign experience as one of the needed qualifications.
"Any reasonable person who reads this job description may come to the conclusion the NDP is using parliamentary resources to hire field workers to prepare for the next federal election campaign," he said in a letter to the internal economy board asking it to investigate.
Andrews also charged that the Montreal office has been used to conduct election training and as a location to drop-off party membership forms.
"All of this together is a disturbing pattern of possible use of parliamentary resources for party activities."
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