Elections Canada is looking into whether Del Mastro, who represents Peterborough, Ont., broke election spending laws in connection with voter-contact calls made by his campaign in 2008.
But despite repeated demands from the opposition benches to step down as parliamentary secretary to Prime Minister Stephen Harper, Del Mastro insisted he's always served with "integrity and conviction" and has done nothing wrong.
"My statements, that were provided in 2008, some four years ago, accurately reflect all expenditures incurred by both campaign and my association," Del Mastro told the House.
"Anything that I paid on their behalf was refunded to me."
The Ottawa Citizen and Postmedia News cited court documents Wednesday which show Elections Canada is investigating Del Mastro for allegedly breaching the spending limit for his 2008 election campaign by more than $17,000.
That would include $21,000 for election expenses, paid for with a cheque drawn on Del Mastro's personal bank account — a sum that would exceed the $2,100 contribution limit for candidates if it was proven to be a personal contribution.
None of the allegations have been proved in court, and Elections Canada refused Thursday to confirm or deny whether an investigation was ongoing.
In an interview with CTV, Del Mastro said he hasn't been contacted by Elections Canada, and hasn't been able to look at the court documents — he was told by court officials the documents are "sealed."
He maintained that he's violated no laws.
"I will have more to come forward with on this matter very soon," Del Mastro said. "But I've got to pull all these records together. I don't walk around with statements and cheques from 2008 in my back pocket."
Del Mastro has been the main Conservative point man in defending the party against allegations of voter suppression in one Ontario riding during the 2011 campaign.
He's earned a reputation for exuberant political bluster and bravado, but evidence of his signature political style was in scarce supply Thursday as he shuffled off the Hill with what one observer called a "hangdog" expression.
NDP ethics critic Charlie Angus said he's known Del Mastro for years and he has no desire to see him punished, calling him a good parliamentarian.
But he said he has lost his credibility as the point man on the Elections Canada file and needs to step aside.
"This isn't about his guilt or his innocence," Angus said after question period. "This is about him being compromised in his role."
But Del Mastro said he hasn't been charged with anything.
"I haven't even been contacted, so maybe they'd also like me to walk the plank with steel plates around my ankles," he told CTV.
Interim Liberal Leader Bob Rae said Del Mastro's self-defence in the Commons made no sense.
"He didn’t indicate for a moment that he even understood the nature of the charges or the nature of the investigation that was underway," Rae said.
"I thought it was a pretty extraordinary performance."
While Del Mastro defended himself against the first volley of questions in the Commons, subsequent rounds were fielded by the prime minister's former parliamentary secretary, Pierre Poilievre.
When other Tory MPs come under scrutiny, it's not uncommon for the government to delegate someone else to answer Commons questions on their behalf. But Angus called Poilievre's leap to Del Mastro's defence odd.
"Is Mr. Poilievre the new spokesman for the prime minister?" Angus wondered. "I think they should just say that and then we can get on, the investigation can happen."
Poilievre tried to turn the tables on the NDP, pointing out they themselves were just slapped by Elections Canada for financing violations of their own.
The party was ordered this week to pay back money collected from unions that sponsored events at last year's national convention. And last fall, the NDP ran afoul of financing laws after soliciting funds destined for a think tank — a parliamentary no-no.
"What we do not yet know, because its leader will not reveal, is how much illegal dirty money did it take and when and how much will it be paying back," Poilievre said.
NDP MP Alexandre Boulerice declined Thursday to reveal the amount.
"The important thing is that we respect the law and the only party that's been found guilty of electoral violations is the Conservative party," he said.
In March, the Conservatives paid back Elections Canada $230,198 and dropped an appeal at the Supreme Court. The case involved national funds funnelled to local ridings that in turn paid for national advertising in the 2006 election — a practice that caused the party to exceed its legal spending limits.
The party also settled last November for another $52,000 over a separate set of regulatory charges related to the so-called "in and out'' scheme.
Currently, allegations that the Conservatives engaged in voter suppression during the last general election are under investigation by Elections Canada.
The Conservatives have acknowledged some irregularities in the riding of Guelph, Ont., but there is no evidence directly linking the party to misleading "robocalls.''