The NDP leader insisted Wednesday there's "not a scintilla" of evidence that New Democrats did anything wrong when they used their taxpayer-funded House of Commons budgets to pay the salaries of workers in satellite party offices.
He accused Conservatives and Liberals of playing political games in a bid to discredit the ascendant NDP.
"It's just our Liberal and Conservative adversaries using their majority to try to frustrate the party that's coming up in the polls and doing well compared to the two old line parties," Mulcair said following the weekly NDP caucus meeting.
However, Trudeau pointed out that the issue has been boiling for "an awfully long time" — well before the NDP's recent rise in the polls. And the Liberal leader noted it was Commons administrators who determined that New Democrat MPs broke the rules — what he called "a very serious independent finding."
"What people expect from their leaders is that when you make a mistake, you own up to (it), you apologize for it and you make it right. You fix it," Trudeau said following his own party's caucus meeting.
"Instead we see Mr. Mulcair trying to deny that anything actually happened and blame others. And that's just not the leadership people need to see."
Later in the Commons, Harper got into the act as he deflected a question from Mulcair about disgraced Sen. Mike Duffy.
The prime minister asserted that Canadians wonder why Mulcair "continues to justify" improperly using $3 million in parliamentary funds and why he's "not being willing to pay it back and not being willing to apologize or reverse course."
Mulcair retorted by rhyming off the names of Conservatives convicted of wrongdoing: "That is what happens when they go to a real court."
The NDP maintains it's the victim of a "kangaroo court," the secretive, multi-party board of internal economy which polices Commons spending.
The board has ordered 68 current and former NDP MPs to repay $2.75 million for the satellite office scheme. It has also ordered them to repay another $1.2 million for using free parliamentary mailing privileges to send out almost 2 million partisan missives.
The NDP has so far refused to repay any of the money and has filed a legal challenge to the board's rulings in Federal Court.
However, the court case has been stalled since both sides agreed last fall to suspend proceedings while they attempted to negotiate a settlement. Those negotiations have gone nowhere and sources say the board's lawyer will move shortly to reactivate the court case.
But Commons administrators aren't waiting for the court to resolve the matter. Sources say they've given New Democrat MPs repeated written warnings that they have until the end of next month to reimburse the money owed or it will be docked from their expense claims.
Mulcair insisted that's not true. But the office of government whip John Duncan, a spokesperson for the board, confirmed it.
"House administration will begin to collect funds from NDP members starting July 1st," Christine Maydossian said in an email. "Administrators will collect the money by refusing to cover expense claims (per diems, travel expenses, etc.)"
On average, each of the 68 New Democrat MPs owes about $20,000. But a few owe more than $100,000.
Should some of them be defeated in the federal election, any money left owing would be docked from their separation pay or — in the case of those defeated before serving six years — from the reimbursement of their contributions to the MPs' pension plan.