MONTREAL -- Liberal Leader Justin Trudeau helped keep the issue of how the Prime Minister's Office handled Sen. Mike Duffy's expenses scandal in the public eye on Sunday, announcing he was sending an open letter to Stephen Harper asking him for more answers.

Harper has refused to provide straight answers about the repayment of Duffy's expenses and who in the PMO knew that former chief of staff Nigel Wright made a $90,000 payment on Duffy's behalf, Trudeau told reporters in Montreal, where he was taking part in the city's Pride parade.

"There are questions about the inconsistencies in his own stories, his own contradictory statements, there are inconsistencies in what ministers of the Crown have been told to say to Parliament," Trudeau said.

In the letter, Trudeau asked Harper who exactly knew about the payment and whether he had spoken to Wright since he left the Prime Minister's Office in 2013.

He also repeated a call he first made on Saturday for Harper to fire some of his closest advisers who knew about the $90,000 repayment scheme before it become public.

 Initially Harper said Wright acted alone, but Duffy's fraud trial heard evidence last week that half a dozen PMO staff and Conservative party brass were in the know about Wright's plan -- including Ray Novak, Harper's current chief of staff.

Harper changed his tune Friday, saying the "vast majority" of his staff didn't know about Wright's actions.

Wright will continue to be cross-examined on Monday at Duffy's trial, where the senator has pleaded not guilty to 31 charges including fraud and breach of trust.

"Canadians deserve answers to these questions -- answers you have refused to provide," Trudeau's letter concluded.

NDP leader Tom Mulcair was also on the attack against Harper over Duffy. The prime minister stuck to his guns when asked about the Duffy affair, saying he did not know about Wright's repayment plan before it became public.

Trudeau also used a media event on Sunday to take aim at his rivals' economic records, declaring Harper's plan "a failure" and accusing Mulcair of taking Quebecers for granted.

"He's offering a minimum wage that most people won't touch, he's offering daycares that are less good than what Quebec already has, and he's offering a mega round of constitutional debates," Trudeau said of NDP leader Tom Mulcair, before reiterating his promises to create jobs, increase family allowances, and cut taxes for the middle class.

Like the other leaders, Mulcair, Gilles Duceppe and Elizabeth May, Trudeau pointed out the conspicuous absence of Stephen Harper, the only national party leader not present at the Montreal Pride event.

"The fact that Mr Harper continues to choose not to attend Pride events -- not just here in Montreal but across the country -- once again shows that he is not choosing to be the Prime Minister for all Canadians," Trudeau said.

Trudeau marched briefly in the parade, chatting with Duceppe and Montreal Mayor Denis Coderre, and stopping to shake hands with a few bystanders.

When asked how he felt about trailing the NDP and the Conservative Party in some recent polls, Trudeau said he remained "very confident" of his party's election chances.

"It's a long campaign, we have work to do across the country -- all the parties -- to attract people's confidence and their votes," he said.

"I am very, very happy with what we're doing," he said.