OTTAWA -- Stephen Harper called any comparison between the Mike Duffy affair and the Liberal sponsorship scandal "absurd" as the Conservative leader faced more questions on Sunday from reporters and attacks from his opponents about the senator.

The Duffy issue stayed in the election campaign spotlight after revelations emerged last week during the senator's fraud, breach of trust and bribery trial.

Harper was reminded Sunday by a reporter how he wouldn't accept claims years ago that former Liberal prime minister Jean Chretien and then-finance minister Paul Martin knew nothing about the sponsorship scandal in the 1990s.

The Tory leader was then asked to explain the difference between that situation and the Duffy affair, which Harper has insisted he knew nothing about.

"Look, in the Liberal sponsorship scandal $40 million of Canadian taxpayers' (money) disappeared," .

"So, I think the comparison is absurd.

"This case we have a senator whose expenses, in our judgment, were not justifiable and my response was that he should pay those expenses back."

Harper's rivals sought to keep the Duffy issue alive on Sunday as the campaign entered its third week.

Liberal Leader Justin Trudeau sent an open letter to Harper, demanding explanations to alleged contradictions and inconsistencies in his statements, and those of some of his cabinet ministers.

In the letter, Trudeau also called on Harper to fire members of his team who were allegedly involved in a Duffy cover up.

"Mr. Harper continues to ignore the calls for answering questions, for explaining why he keeps these people who have been involved in part of a cover up to hide the truth from Canadians -- why he continues to keep them around him, indeed, running his campaign," Trudeau told reporters in Montreal, where he attended a Pride event.

Last week, Harper said the "vast majority" of his staff were unaware of a scheme to fake Duffy's personal repayment of Senate expenses. The statement marked a shift from Harper's previous position that only one person was aware.

One of the people informed about the plan by Harper's then-chief of staff, Nigel Wright, to repay Duffy's expenses was Ray Novak, Harper's current chief of staff, close confidante and a senior Conservative Party campaign director.

On Thursday, Harper supported Novak's claim that he did not read a direct email on the subject from Wright -- his boss at the time -- and was unaware about Wright's payment.

New Democrat Leader Thomas Mulcair was asked Sunday whether some of Harper's staffers should lose their jobs over the Duffy scheme.

"Well, I think somebody should be fired for the whole Duffy-Wright affair," Mulcair said in Montreal, where he also attended the Pride event.

"On Oct. 19, Canadians will get a chance to fire that person responsible for that whole mess -- and that is Stephen Harper."

Mulcair also took a shot at Harper for "systematically" staying away from gay pride parades in Canada.

"It's a way of showing that you respect rights and that you want to remove discrimination," he said.

Mulcair was also asked about Green Party Leader Elizabeth May's announcement Sunday that former New Democrat MP Jose Nunez-Melo had decided to run under her banner in a new riding in suburban Montreal.

Montreal's La Presse newspaper reported this month that Nunez-Melo got into a dispute with the party over the nomination in the new district. The report said Nunez-Melo alleged party brass blocked him from running.

Mulcair said Nunez-Melo informed the party in a letter that he didn't accept the rules.

"And the party said, 'Fine, you're not a candidate,' " he told reporters.

"If you don't accept the rules, you can't be a candidate."

May said her party now has three candidates who sat as MPs in the last Parliament, which was dissolved earlier this month.

"I am over the moon to welcome Jose Nunez-Melo," May said. "We weren't in the same party but we had the same spirit."