Conservative Leader Stephen Harper fielded more questions about the Mike Duffy affair while on the campaign trail Sunday, particularly over the apparent involvement of his current chief of staff Ray Novak.

Evidence presented during Senator Duffy's criminal trial this week suggested that Novak, then Harper's deputy chief of staff, may have been aware of Nigel Wright's secret plan to repay Duffy's questionable expense claims with a $90,000 personal cheque.

An Ottawa court heard on Thursday that, in an email sent on March 23, 2013, Wright — Harper's chief of staff at the time — wrote to Novak and the lawyer for the Prime Minister's Office saying, "I will send my cheque on Monday."

Though the email suggests Novak was aware of Wright's plan to cover the expenses, Harper continued to say publicly that Duffy had repaid the money himself — until May 2013 when Wright's involvement was revealed.

Earlier this week, a Conservative spokesman said that Novak never opened the email from Wright because he had been pulled away for a conference call.

Emails filed as evidence in the criminal trial of Senator Mike Duffy suggest that Ray Novak, Conservative Leader Stephen Harper's current chief of staff, was aware of Nigel Wright's plan to write a $90,000 cheque to repay Duffy's questionable expenses. (Adrian Wyld/Canadian Press) At a campaign stop in Ottawa on Sunday, CBC News reporter Hannah Thibedeau asked Harper why Novak hadn't been reprimanded for "knowingly" allowing the Conservative leader to "repeat a lie for months" that Duffy had repaid his own expenses.

"What I said very clearly is that I was told that Mr. Duffy had repaid his own expenses. That's what I said should happen," said Harper, prefacing his answer by saying he didn't "accept the premise" of the question.

"When I found out that Mr. Duffy had not paid those expenses and that Mr. Wright had paid them instead, I held those two individuals responsible for their actions and we are holding them accountable," he added to applause from the crowd.

Asked if the Canadian public should believe that Novak had not read the email and was unaware of Wright's intentions, Harper repeated that the "two individuals responsible are the ones being held accountable."

Harper's answers largely echoed statements he gave to media on Friday.

Comparison to sponsorship scandal 'absurd,' Harper says

Three of the 31 charges that Duffy faces are related to the famous $90,000 payment that he received from Wright — he has pleaded not guilty to all the counts, which include fraud and breach of trust.

Wright moved out of his position after it became public that he gave the senator the money.

Wright testified last week that he provided the money to Duffy after an original plan, that would have involved the Conservative party fund paying back the expenses, fell through.

Reporters ask NDP leader about PMO staffers who knew of Mike Duffy payment 0:45

During the question and answer session Sunday, Harper also scoffed at any comparison of the Duffy affair to the sponsorship scandal of the former Liberal administration.

"I think the comparison is absurd," said Harper, who noted the sponsorship scandal involved tens of millions of dollars.

"In this case we have a Senator whose expenses in our judgement were not justifiable, and my response was he should pay those expenses back and that's what I was told happened."

Also in attendance at Harper's campaign event was former Conservative senator and former government leader in the senate Marjory LeBreton. LeBreton played a central role in the initial efforts to contain the fallout from the Duffy affair and was prominently featured during campaigns in the past.

When reporters approached LeBreton to pose questions about Duffy's ongoing trial and Wright's testimony, a Conservative campaign worker quickly intervened and whisked her away from the cameras.

Mulcair, Trudeau weigh in

Speaking to reporters at the Montreal Pride parade on Sunday, NDP Leader Tom Mulcair said he's "not going to start running after subordinates" and that Harper should ultimately be held responsible for the Duffy affair.

Liberal leader says he will be sending an open letter to Stephen Harper, asking him specific questions that he's refused to answer from the media 1:28

"I do think that someone should be fired for the whole Wright-Duffy affair and it's the person who named Mike Duffy to the Senate — the same person who hired that chief of staff, the person in charge of the Prime Minister's Office when the $90,000 cheque was given — that's the prime minister," said Mulcair.

Liberal Leader Justin Trudeau, also speaking at Montreal's Pride event, repeated his call for Novak and other senior Conservative aides to be fired.

Beyond the leaders' statements, both the NDP and Liberals tried to keep the expense scandal on the forefront of voters' minds over the weekend.

On Sunday afternoon, Trudeau released an open letter to Harper, in which he said some of the evidence tabled in the Duffy trial "is raising extremely serious questions about your judgment and your public statements to this point."

For their part, the NDP unveiled a tongue-in-cheek video on its Facebook page of children reading a selection of the emails revealed at the trial. Based on some comments posted to the video, it did not resonate with everyone, with some users criticizing the inclusion of children in a political message.​

Read the open letter Trudeau addressed to Harper regarding the Duffy affair: