BURNABY, B.C. -- Elizabeth May is accusing NDP Leader Tom Mulcair of giving cover to Stephen Harper by refusing to participate in debates his Conservative rival does not attend.

The Green party leader said it's shameful and shocking that Harper -- and now Mulcair -- won't show up for a scheduled debate on women's issues, which has since been cancelled as a result.

May said the best way to hold Harper's feet to the fire is for all party leaders to attend and leave an empty podium to indicate the prime minister's absence.

"Call his bluff. He'll show up," she said Tuesday while unveiling her party's national housing strategy in Burnaby, B.C.

"Instead of calling his bluff, Tom Mulcair is helping him out. If the debates don't happen, Stephen Harper's off the hook, but Tom Mulcair will wear this and I'm begging him to reconsider."

May, the only major female party leader, said it's not "smart politics" for Mulcair to dismiss a women's issues event while participating in debates to which she has not been invited.

The debate's organizers, Up for Debate, said they will instead pre-record one-on-one interviews with leaders and release them at a Sept. 21 event in Toronto.

Mulcair said he supports that alternative because it will allow for a more complete discussion.

"I would have certainly preferred the full debate format with the prime minister present, but barring that, I think this is the best we can do under the circumstances," he said Tuesday.

Up for Debate spokeswoman Kelly Bowden said questions are being crowdsourced and will explore issues including violence against women, economic equality, leadership and political participation.

"We're still confident that the alternative format we're proposing will create a space where leaders can address head-on questions that are posed by and related to women in the election," Bowden said.

The discussion is essential, she added, because women, while not homogenous, are affected uniquely by certain election issues, including child care.

Cristine de Clercy, an associate professor of political science at the University of Western Ontario, said parties must be careful about how they handle the child care issue.

"They are sending signals out about their position on some big issues around women's access, what does work count for, all these things that they have to be careful about anyway," she said.

Liberal leader Justin Trudeau tweeted on Monday that he would support a televised discussion on women's issues with May.

"Hope you'll reconsider joining us," he tweeted to Mulcair and Harper.

Conservative campaign spokesman Kory Teneycke said Harper -- invited to dozens of debates -- picked five, including one sponsored by the Globe and Mail and Google Canada on Sept. 17 and a French-language debate on Quebec's TVA network on Oct. 2.

He declined to comment on debates that Harper was not attending.

"We've picked five that we think are the right five, that represent issues of the most interest to Canadians of the dozens," he said Tuesday.

"It's not really a criticism of that particular debate or any of the others, we're just not commenting on ... each one specifically, because there's way too many of them."

May announced on Tuesday her party's plan to retrofit all homes by 2030 to increase energy efficiency, implement a guaranteed livable income and increase social housing for First Nations.

She also vowed to eliminate a Conservative government program that gives permanent residency to millionaire foreign investors.

-- With files from Stephanie Levitz in Ottawa, Allison Jones in Hamilton, Andy Blatchford in Quebec City and Murray Brewster in Toronto