Ontario PC leadership candidate Caroline Mulroney sought to explain her shifting position on a provincial carbon tax Friday, saying she was never entirely comfortable with former leader Patrick Brown's insistence on using this sort of revenue tool in the first place.

Mulroney, the only person in the race for the Progressive Conservative leadership who is also a nominated candidate in a provincial riding, attributed her initial reluctance to speak out against the carbon tax to the fact that she's been selling it to voters in the riding for months.

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Brown's platform, unveiled last fall, was full of new spending promises. It relied on the estimated $4 billion in new revenue a carbon tax would bring in to pay for those commitments.

"Since November, I have been part of the PC Party team knocking on doors, talking about our platform, which includes a carbon tax. But you know, the carbon tax never sat well with me, and now that I'm putting my name forward, and I'm developing my own plan, I was clear. I oppose all new taxes, including a carbon tax," Mulroney said in an interview with CBC's Power & Politics.

"We'll be fighting the carbon tax ... Ontarians are already paying too much, life has become unaffordable," she said. "There will be other ways we will be able to protect the environment."

On Monday, during a fireside-style chat with the deputy leader of the federal Conservatives, Lisa Raitt, Mulroney described the carbon tax proposal as a sort of necessary evil, since the federal Liberal government is determined to impose carbon pricing on all provinces that don't have plans of their own. The PCs could go ahead with the tax and "make sure that we ... put the money back in people's pockets," she suggested.

She also said she would consult with party members to see where they stand. Only two days later, she said a carbon tax would be a non-starter in a future Mulroney government.

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The other candidates in the race to replace Brown — former Toronto city councillor Doug Ford and former Ontario MPP Christine Elliott — also have said they don't support a carbon tax.

"It's just a tax grab," Ford said. "Carbon tax in my opinion is a bad tax. It's a bad tax for businesses, it's a bad tax for people."

During an address to the Manning Networking Conference in Ottawa Friday, Conservative Leader Andrew Scheer said he was happy to see all three candidates for the PC leadership line up against a price on carbon — a policy he has often railed against in the House of Commons as too costly and a threat to the competitiveness of the country's natural resources sector.

"That is great news from our movement, that is great news for Canadians, and I can assure you as prime minister, in 2019, the first act that Canada's Conservatives will do is repeal the carbon tax," he said to applause.

Legal options

On Friday, Mulroney said she would be willing to partner with other provinces opposed to the federal plan to pursue legal action against Ottawa over the imposition of a carbon tax.

"We're exploring all the legal options ... there are other provinces looking at it. My team is looking at exactly that, and as premier I will continue to do that," she said.

Former Saskatchewan premier Brad Wall floated legal action as a way to stop a carbon tax.

The PC government in Manitoba also considered such an idea before receiving independent advice that warned it would likely be a fruitless exercise, as Ottawa is within its rights to levy a tax.

"If we just say 'no,' we get Trudeau. If we go to court, we lose," Manitoba Premier Brian Pallister said after legal scholar Bryan Schwartz delivered his report on the constitutionality of the federal plan.

"Our alternatives are pretty clear, I would say. We develop our made-in-Manitoba plan and we put it out there."

When asked about Manitoba's decision to not pursue a court challenge, Mulroney said she's still open to legal action if elected premier.

"If other provinces get on board then it's the federal government that is losing the battle," she said.

Under the plan drafted by Prime Minister Justin Trudeau's government, Ottawa will impose a tax of $10 a tonne in 2018, rising by $10 each year to $50 a tonne by 2022, on any provinces that don't have either a carbon tax or a cap-and-trade-style system.

Trudeau has said the plan will be revenue neutral for the federal government, as any money generated under the system will be returned to the province or territory where it's generated.

Wasteful spending

When asked Friday how she would pay for new campaign promises without the tax — and Mulroney has said there is much to like in the People's Guarantee, the name Brown gave to his plan — she said the Liberals have been so wasteful she's confident she can find places to save money.

Mulroney pointed to a recent report by the provincial auditor general that found some $1 billion had been misspent. "We will look at ways to run our government more efficiently ... I'm confident we will find ways."

Mulroney said her campaign team is working on a fully-costed platform that will be released before PC party members cast their votes.

With a file from the CBC's Mike Crawley