Liberal Leader Michael Ignatieff says new revelations about past fraud convictions against a former senior aide to Stephen Harper raise a "fundamental question" over whether Canadians can trust the Conservative leader.

During a Monday evening campaign rally, Ignatieff told the crowd that Bruce Carson's close relationship with Harper while working in the Prime Minister's Office shows his Conservative rival "talks tough on crime everywhere but in his own office."

"I'm always willing to give a man another chance," Ignatieff said. "But he had five convictions for fraud and he was in the inner circle of the prime minister.

"This is the fundamental question in this election. Can you trust this prime minister with power? Can you trust him to respect our democratic institutions?"

Carson worked as a senior adviser in the Prime Minister's Office until 2008. An investigation into Carson's business dealings by the Aboriginal Peoples Television Network led the PMO to call in the RCMP in March. Since then, Carson's past fraud convictions and bankruptcies have surfaced in the news.

Earlier Monday, Harper said if had he known about the latest revelations surrounding his former aide, he wouldn't have hired him.

"I'm learning these things as well," Harper said at a campaign event in southern Ontario. "Had I known these things, obviously I would not have … hired him."

But Carson's lawyer told CBC News Monday that Carson was upfront about his fraud convictions during a required security screening.

Patrick McCann, who represented Carson during his past legal troubles and spoke to him Monday, said Carson doesn't have the security clearance application, but he remembers telling them everything.

"He honestly believes that everything was disclosed in his application for security clearance," McCann said. "All his criminal convictions, in other words."

3 more convictions emerge

The Canadian Press reported Sunday that Carson had three more convictions than previously known, and that he had been sentenced to six months of psychiatric care in 1990.

Carson told The Canadian Press he recalls mentioning his criminal past in early 2006 to Ian Brodie, then Harper's chief-of-staff, when applying for Secret-level clearance.

"I remember going to him and saying to him, 'I've got to fill out these forms," Carson said. "You know I have a criminal past. Should I go ahead and fill them out, or is that sort of the end?' He said, 'No, go ahead and fill them out,' and so I did."

Brodie said Monday he first heard of Carson's most recent criminal convictions in the news, The Canadian Press reported. "I do not think Prime Minister Harper would have been aware of these more recent charges, either," Brodie said.

Carson also gave Harper the benefit of the doubt, according to The Canadian Press. "He's the prime minister. He wouldn't be reviewing everybody's application for a security clearance."

Carson's lawyer said he couldn't comment on Carson's reaction to what Harper said earlier in the day, but "he's certainly not happy about anything that's going on right now."

'Difficulties with the law'

During a Monday campaign stop in Ontario, Harper said he knew Carson had had "difficulties with the law many, many years ago" and that after that time, he had a solid employment record including various jobs on Parliament Hill and he developed a good reputation.

"He was well-regarded. On that basis, the Privy Council Office gave him a security clearance," said Harper. "The fact is, I did not know about these revelations that we're finding out today. I don't know why I did not know."

The Conservative leader said his office and the Privy Council Office will have to review their procedures "to make sure these things get caught."

A spokesman for PCO — the bureaucratic arm of the PMO — says the department is always assessing its internal procedures, but wouldn't say whether they are specifically reviewing the security clearance process. He also wouldn't say whether the office has checked to confirm Carson's assertion.

The PMO asked the RCMP last month to investigate Carson after a probe by the Aboriginal Peoples Television Network alleged the 65-year-old may have illegally lobbied the federal government on behalf of a company that employed his girlfriend, a 22-year-old one-time escort.

In commenting on the newest revelations about Caron's criminal record, Harper said it's important to remember that his former staffer is not accused of anything that has to do with his employment in the Prime Minister's Office.

With files from The Canadian Press