Questioned relentlessly at every campaign stop about the fallout from the Mike Duffy trial, Stephen Harper is refusing to be knocked off his double-barrelled core campaign message: economy and security.

The Conservative leader is stressing the latter at a stop in Fredericton, N.B., where he is promising to add 6,000 people to bolster the reserve ranks of the Canadian Forces reserves.

Harper says the measure will cost $163 million over three years and $63.4 million going forward once the overall target of 30,000 personnel is reached.

His main opponents, meanwhile, want heads to roll over the Duffy affair. Liberal Leader Justin Trudeau wants staffers in the Prime Minister's Office fired; NDP Leader Tom Mulcair says it's Harper who should be turfed.

But the prime minister says the two people to blame are Duffy and Nigel Wright, Harper's former chief of staff, the star witness at Duffy's trial and the man who personally paid the embattled senator's questioned expenses.

Harper — ignoring evidence that indicates a number of PMO staffers were aware of the arrangement — says Duffy and Wright were the two principal players and are the ones being held accountable.

'Not a lie': Wright

Mulcair was in Niagara Falls on Monday, where he noted that inconsistencies with Harper's public comments are starting "to add up."

"Mr. Harper has not been truthful with Canadians, that has become abundantly clear from the emails that have been released and Canadians deserve better."

Trudeau was campaigning in Ajax, Ont. where he was asked about a letter he wrote to Harper Sunday asking him to answer a list of questions he has refused to answer in public.

"Canadians... are very concerned with the fact that the Prime Minister's Office and his senior staff were involved in a cover-up," Trudeau said. "The fact is these are very serious questions that the prime minister must answer."

Wright, the prime minister's former chief of staff, was in an Ottawa court again on Monday where he was cross-examined by Duffy's lawyer, Donald Bayne.

Duffy has pleaded not guilty to 31 charges of fraud, breach of trust and bribery related to expenses he claimed in 2013 as a senator and later repaid with money from Wright.

Before Wright gave Duffy $90,000 to repay his ineligible expenses, there had been an initial plan to see Duffy repay part of his expenses with money from the Conservative Party fund.

But Wright testified he did not tell Harper about either plan.

Bayne asked Wright Monday why he would lie to the prime minister.

"I don't think I lied to the prime minister," Wright said. "I don't feel it was lie. I just felt it wasn't on my list of things I needed to check with him."