But there are holes in both the NDP's lines of defence.
Documents obtained by The Canadian Press suggest it was neutral House of Commons administrative officials who determined the NDP mailings violated the rules for using MPs' Commons envelopes and free postal privileges.
And it appears Elections Canada may now be investigating some of the mailings that arrived in four ridings in the midst of byelections — although the independent agency initially said the missives did not count as campaign expenses because they were mailed before the byelections were officially called.
An "information note" compiled by the House of Commons administration suggests the Conservatives and Liberals acted on the advice of neutral officials in concluding the NDP must repay parliamentary resources used for at least some of the mailings.
"Together these documents demonstrate an activity orchestrated on behalf of a political party for which House resources ... were used," says the analysis of the NDP mailings prepared for the secretive, multi-party board of internal economy, which oversees Commons spending.
"In short, it would appear that the mailings were not messages from the individual members as members, but rather were prepared by and for the benefit of the NDP as a political party and to advance electoral purposes."
Administrative officials came to that conclusion after reviewing samples of 1.8 million pieces of mail sent by 23 New Democrat MPs — including NDP Leader Tom Mulcair — into 26 ridings currently held by other parties. They all used Commons envelopes and free parliamentary mailing privileges, known as franking.
The board has asked the Commons administration to take a closer look at other NDP mailouts to determine whether they also were against the rules.
In early April, Commons Speaker Andrew Scheer, who chairs the board, asked Elections Canada to take another look at the mailings that arrived in the four ridings during byelections late last year.
Liberal MP Marc Garneau, who personally lodged a complaint about the NDP mailings, said it appears Elections Canada is now taking the matter more seriously than it did when it initially dismissed the matter in March.
He said he was questioned on April 29 by two investigators from the commissioner of elections, including Ron Lamothe, the lead investigator who nailed the Conservative party for violating election spending rules through the so-called in-and-out scheme.
"My sense of that was that they were definitely going to follow it up," Garneau said in an interview.
The commissioner of elections refuses to confirm or deny if an investigation has been launched.
But in a May 5 letter to Scheer, chief electoral officer Marc Mayrand also struck a more serious tone, describing "complex matters that warrant careful consideration as they can have very significant repercussions on the activities carried out by Members of Parliament."
Mayrand added that it's important for Elections Canada officials to continue collaborating with the office of the Commons law clerk to provide "clear guidance" to MPs, "especially as we approach the next general election."
Nevertheless, New Democrats insisted Tuesday that they broke no rules and are the victims of a partisan gang-up. Indeed, they maintained they checked with both the Speaker and House administrators before they embarked on the mass mailings.
Scheer has denied he approved any specific set of mailings. Over the objections of the NDP, a Commons committee voted Tuesday to call the clerk of the Commons, Audrey O'Brien, to testify on the matter.
NDP House leader Peter Julian said the Conservatives and Liberals have transformed the once consensual board of internal economy into a "kangaroo court."
"Ironically, this transformation is the ultimate misuse of parliamentary resources being misused for partisan purposes."
The NDP released numerous examples of mailings sent out by Conservative and Liberal MPs, some of which are more blatantly partisan than the missives New Democrats have sent.
One, from Conservative whip John Duncan, compares the "strong economic leadership" of Prime Minister Stephen Harper against the "reckless spending and higher taxes" of Mulcair.
New Democrat MP Pat Martin said 10 Conservative MPs, including four cabinet ministers, have bombarded his Winnipeg riding with "hyper-partisan" mailings. The board of internal economy found nothing wrong with those mailings but now wants the NDP to cease sending out its own.
"The Conservatives want the right to be able to carpet bomb my riding with propaganda but they want a unilateral disarmament (by the NDP)," Martin said.
Duncan, a member of the board, insisted the board's findings are "very fair" and said the NDP "should be accepting responsibility instead of impugning everybody else, including the Speaker."
If the NDP has to pay for the mailings, it could run into the millions of dollars. Duncan refused to speculate on the final bill, but he said there are ways to recoup the money if the NDP refuses to pay.
Follow @jmbryden on Twitter
Also on HuffPost