Mulcair spoke to CBC Radio's Anna Maria Tremonti Tuesday ahead of his appearance at the Economic Club of Canada in Toronto. In the interview, which aired Wednesday morning on The Current, Mulcair touched on his party's economic plan, child care plans and the effort to get its message out as Canada readies for its next federal election.
When asked about corporate taxes, the official opposition leader said his party would increase them in a bid to get "closer to the G7 average."
He said the G7 average is "somewhere in the 18 or 19" per cent range and that Canada is currently "12, 13, something like that, right now."
The federal corporate tax rate has been 15 per cent since 2012, down from 18 per cent in 2010.
Mulcair didn't provide specifics on how much the corporate rate would change if his party was in government and didn't say precisely how long it would take to phase in any changes.
"We're going to go up in increments and we're going to do it responsibly and we're going to do it in a way that doesn't hurt Canadian businesses," he said.
Speaking to reporters on Wednesday, Mulcair said he takes responsibility for any "lack of clarity."
"So I'll take responsibility as I always tend to do for any lack of clarity there, that would be my fault," he said.
Harper, in what was likely his last question period appearance before the federal election, pointed out the error and said "we shouldn't expect the leader of the NDP to know his facts."
"Yesterday, of course, he was out there saying businesses need to pay higher taxes," Harper said. "Then asked what is the tax rate exactly, well, he didn't know and stated it three points lower than it actually is."
The gaffe also drew attention from Finance Minister Joe Oliver.
"He doesn't even know what the tax rate is and yet he wants to raise it," Oliver said Wednesday. "And that seems to be their default position. They just want to continue to raise taxes."
In his speech to the Economic Club Tuesday, Mulcair outlined the support his party would provide to manufacturers, small businesses and municipalities for infrastructure, and touted the New Democrats' record of surpluses when in power provincially.
The NDP leader, who has a memoir coming out in August, said in the interview that his party is working to explain both his own background and experience and they party's priorities and plans.