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

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

Harper said 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.

NDP Leader Tom Mulcair, however, kept the Duffy affair on the front burner, saying Harper misled Canadians about the repayment of the disgraced senator's questioned expenses.

"What Canadians do take away is that their prime minister has not been forthcoming with them, because he's been saying one thing and its opposite and expecting people to believe both can be true strains credibility," Mulcair said at a stop in Niagara Falls.

But the prime minister stuck to his script; 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.

"I do think that what's important for Canadians looking forward is going to be the two issues of this campaign; the economy of our country and our future prospects in the security of our country," Harper said.

"Those are the two issues and this Conservative party is the only government that has answers."

Mulcair used tourist-friendly Niagara Falls to roll out a promise of $30 million over three years to help Destination Canada attract more American visitors. He said the Conservatives have cut the Crown corporation's budget even as tourist numbers have fallen

In Ajax, Ont., Trudeau promised a tax break for the middle class, saying those earning between $44,700 and $89,401 a year will see their tax rate fall to 20.5 per cent.

"We will raise taxes on the wealthiest one per cent of Canadians," Trudeau said.

The Conservatives have neglected the middle class, but that would end under a Liberal government, Trudeau told a crowd in Ajax, one of the vote-rich communities on the Toronto outskirts.

A Liberal government would also retool Canada's child-benefit system to provide more to those families that most need the help, he added.

He said his tax-free child benefit will be fairer than the universal benefit touted by Harper.